ubrd50

re r

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

surface. . . .. . . .. . . .

surfaces compared tosion force of the frost

Advances in Colloid and Interface Science 210 (2014) 4757

Contents lists available at ScienceDirect

Advances in Colloid an

.ethe mitigation of frost formation have been substrate heating andanti-icing chemicals, which are both cost-intensive and environmental-ly problematic [5]. In the automotive industry, frost not only causes

layer is lower on the superhydrophobic surface [8]. Additionally, it hasalso been reported that superhydrophobic surfaces can repel impactingsupercooled water droplets before ice nucleation occurs [11], and thatsures to avoid frost formation and accumulation on surfaces which areexposed to cold environments. So far, the main applied measures for

frost deposition is delayed on superhydrophobica plain metal surface [810], and that the adhecioeconomic costs, due tomaterial damage, energywaste, andmeasuresfor the prevention of hazards [1,2]. Numerous commercial sectors areaffected by the negative impact of frost formation, such as the automo-tive, aerospace, railway, telecommunication, and power engineeringindustries [1,3,4]. Therefore, an urgent demand exists to develop mea-

[6,7].One of the more interesting candidates that are currently being in-

vestigated for anti-icing applications is the use of superhydrophobiccoatings [1]. Various studies have reported the different anti-icing prop-erties of superhydrophobic surfaces. For example, it was shown thatmassive energy wastage due to heating of sweather, but it also represent a hazard due t

Corresponding author. Tel.: +61 8 8302 5719; fax: +E-mail address: drew.evans@unisa.edu.au (D. Evans).

0001-8686/$ see front matter 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.cis.2013.10.018singmassive ongoing so-

the front and rear screens, and sidewindows andmirrors [4]. Therefore,a genuine market demand exists for passive frost prevention measuresand an intense effort has been placed into researching possible solutionsFrost formation in cold environments is cau4. Condensation on superhydrophobic5. Conclusions and outlook . . . .Acknowledgements . . . . . . . . .References . . . . . . . . . . . . .

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572. Nucleation of condensate and ice . . . . .3. Freezing of droplets . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .ContentsKeywords:Anti-icingCondensationFrost retardationSelf-seedingSuperhydrophobic

shown that as well as having ultra water repellency the surfaces have reduced ice adhesion and can delay waterfreezing. However, the structure or texture (roughness) of the superhydrophobic surface is subject to degrada-tion during the thermocycling or wetting process. This degradation can impair the superhydrophobicity andthe icephobicity of those coatings. In this review, a brief overview of the process of droplet freezing onsuperhydrophobic coatings is presented with respect to their potential in anti-icing applications. To supportthis discussion, new data is presented about the condensation of water onto physically decorated substrates,and the associated freezing process which impacts on the freezing of macroscopic droplets on the surface.

2013 Elsevier B.V. All rights reserved.Condensation and freezing of droplets on s

Linda Oberli a,b, Dean Caruso b, Colin Hall b, Manrico Faa Surface Science and Technology, ETH, Zurich, Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerlanb Thin Film Coatings Group, Mawson Institute, University of South Australia, Mawson Lakes, SA

a b s t r a c ta r t i c l e i n f o

Available online 23 October 2013 Superhydrophobic coatings a

j ourna l homepage: wwwurfaces exposed to coldo poor visibility through

61 8 8302 5639.

ghts reserved.perhydrophobic surfaces

etto b, Peter J. Murphy b, Drew Evans b,

95, Australia

eported as promising candidates for anti-icing applications. Various studies have

d Interface Science

l sev ie r .com/ locate /c i ssuperhydrophobic surfaces show little ice adhesion both in laboratoryexperiments with supercooled water spraying and under naturally oc-curring freezing rain [12,13]. Experimental observations such as theseindicate the applicability of using of superhydrophobic coatings as pas-sive elements in anti-ice applications. In recent literature, the termicephobicity has been introduced, which differentiates the properties

of a surface from one that is superhydrophobic [14]. Importantly, a sur-face that is superhydrophobic is not always icephobic [15].

To understand the inuence of superhydrophobic surfaces on frostformation, the characteristics of a superhydrophobic surface have tobe dened. Superhydrophobic surfaces are qualitatively dened bytheir excellent water repellency, and quantitatively by a water contactangle equal to or greater than 150 and a low contact angle hysteresis(typically less than 10) [8,16]. The contact angle of a liquid droplet onan ideally at and chemically homogeneous surface is given by theYoung's relation [17]:

cos SVSL =LV 1

where =contact angle (); and =interfacial surface tension (J/m2).The subscripts refer to S=solid, V=vapour, and L=liquid. Young's re-lation describes the equilibrium of all three interfacial tensions involvedat the triple point of solid, liquid, and vapour, of a liquid droplet on a

During experimental observations of contact angles for liquid drop-lets on solids it was noted that when the surface possessed physicalroughness, rather than being smooth, larger contact angles were mea-sured on hydrophobic surfaces and smaller contact angles on hydrophil-ic surfaces than that predicted by Young's equation.Wenzel rationalisedthis through the denition of a correction factor, which takes into ac-count the roughness of the surface. In theWenzel equation a correctionfactor is introduced for the contact angle, which is equal to the ratio ofthe actual contact area between a liquid droplet and a solid surface tothe geometric contact area of a corresponding smooth surface [21,22]:

cos Rcos 3

where R = Arough/Asmooth =Wenzel correction factor; = correctedcontact angle (); and A=contact area between liquid droplet and sur-face (m2). TheWenzel equation is valid and is used to describe physical-ly rough but chemically homogeneous surfaces on the assumption that

48 L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757smooth solid surface. Hydrophobic surfaces have a low surface free ener-gy and produce a contact angle with a water droplet that is higher than90. Hydrophilic surfaces, on the other hand, have a high surface free en-ergy and produce a contact angle with awater droplet that is lower than90, and is therefore water attracting. The contact angle of a waterdroplet on a hydrophilic, hydrophobic, and superhydrophobic surfaceis illustrated in Fig. 1.

The adsorption of water onto a surface is caused by the balance be-tween adhesive and cohesive forces. As the adhesive force increases rel-ative to the cohesive force, the water increases its contact area with thesubstrate (therefore reducing its contact angle). Hydrogen bonding andthe relative strength of the hydrogen bonding between the water mol-ecules and the substrate dictate the magnitude of the adhesive force.Hence, hydrogen bonds on a substrate act as sites for water absorption.Hydrophobic surfaces have hydrogen bonding sites with low bondingstrength, leading to the cohesive force between water molecules beingstronger than the adhesive force of the water on the substrate. In con-trast, hydrophilic surfaces have hydrogen bonding sites with strongbonding strength such that the adhesive force is greater than the cohe-sive force [18].

The static contact angle hysteresis is dened as the difference be-tween the advancing and the receding contact angle of a liquid dropleton a surface [19]:

ar 2

where =contact angle hysteresis (); a=advancing contact angle(); and r = receding contact angle (). It is worth noting however,that contact angle hysteresis can also be inuenced by chemical andphysical surface heterogeneities, and droplet velocity in dynamiccases [20].Fig. 1. Schematic representation of a water droplet on a hydthe droplet completely wets that surface. By way of contrast, a hydro-phobic surface with sufcient roughness can remain partly non-wetting under a liquid droplet. For such a situation the liquid dropletnow sits on a mixture of solid areas and air pockets, and theWenzel re-lationship becomes invalid [5,12,23]. To calculate the contact angle forsuch a non-wetting state a derivation of the CassieBaxter equation isapplied, which takes into account the fact that the droplet is in contactwith both the substrate and air. The overall contact angle takes into ac-count the relative contribution from the substrate and air. In the hydro-phobic case the following equation is applied [24]:

cos 1 S cos 1 4

where S = solid fraction of the substrate in contact with the liquiddroplet. The parameter S is commonly referred to as the solid frac-tion, and can be approximated as the area of the peaks of the solidsubstrate's asperities as a fraction of the total geometric area. Bicoet al. indicated that surfaces with low roughness (R=1.3, where a atsurface has R=1.0) could be made strongly hydrophobic with contactangles as high as 170 being measured [24]. A recent review article onthe subject of wetting and roughness discusses the subtleties and limi-tations of Eq. (3), pointing to the fact that perfect wetting or drying isnot achieved in the limit of R1 [25].

The aforementioned models are useful in describing the wettingstate of a water droplet resting on a surface, however, Eqs. (3) and (4)are in essence incorrect since it is the surface structure experienced bythe three-phase contact line and not the surface structure under thedroplet which ultimately determines the contact angle [26,27]. Despitethis conict, in many practical cases these equation however work wellowing to the three-phase contact line experiencing the same surface to-pography as that found under the droplet. As conceptual models torophilic, hydrophobic, and superhydrophobic surface.

understand different wetting state (i.e. complete or incomplete wet-ting), those presented above are sufcient to explain wetting phenom-ena, even if Eqs. (3) and (4) cannot be explicitly employed. In relation toice nucleation (the specic focus of this discussion) it is the wettingstate rather than the contact angle which is of most interest, andhence the models are thus justied here.

A droplet in the non-wetting CassieBaxter state has less intimatecontact with the solid surface than a droplet in the fully wettingWenzelstate. As a general consequence this produces a lower contact angle hys-teresis, and roll-off angle [5,28]. This roll-off angle is dened as the angleat which a droplet of a certain weight spontaneously starts to slidedown an inclined surface [9]. The reduction in solidliquid contactarea (solid fraction, S) has been used to rationalise the longer freezingtimes of droplets on roughened surfaces, for example in the case of

L. Oberli et al. / Advances in Colloid and Inanostructured versus at surfaces [29]. A static droplet in a wettingWenzel and a non-wetting CassieBaxter state is illustrated in Fig. 2. Itthen follows that droplets of water placed on a superhydrophobic sur-face are typically in the CassieBaxter state [28].

Out of theWenzel and the CassieBaxter state, the onewith the low-est energy is the more stable state, though this low energy state doesnot necessarily imply the lowest contact angle. The critical contactangle for the transition from the Wenzel to the CassieBaxter state forhydrophobic surfaces arises from equating Eqs. (3) and (4), and is de-ned as:

cosC S1 = RS 5

where C=critical contact angle ().For hydrophobic surfaces the CassieBaxter state is energetically

more stable for contact angles greater than C [30]. However, under cer-tain circumstances a transition between the non-wetting CassieBaxterand the wetting Wenzel state can occur. Wetting can be initiated atstructural surface defects, under increased pressure conditions, or dueto condensation of water [5,6,28,31]. If the Wenzel correction factor Rand/or solid fraction S increases, C becomes smaller (and approaches90o) and therefore the non-wetting CassieBaxter state is more proba-ble than the wetting Wenzel state. Thus, for the design of super-hydrophobic coatings in the non-wetting CassieBaxter state, asurfacematerial with low surface free energy and a suitable topographyare required. A suitable topography implies sufcient roughness and anoptimised solid fraction [2].

The application of superhydrophobic surfaces as anti-icing coatingsis limited by two severe problems, namely condensation and wear. Invarious studies it was observed that in cold and humid environmentscondensation of water droplets was occurring on superhydrophobicsurfaces [1,2,32]. Condensed water can inltrate rough structures,resulting in awettingWenzel state surface. The contact angle hysteresisis then increased due to surfacewetting and the superhydrophobicity issubsequently lost [1,2,6,32]. Upon freezing of the condensed waterwithin the surface structure, the resulting ice becomesmechanically an-chored to the surface and larger ice adhesion forces arise. Therefore, ice

Fig. 2. Schematic illustration of a water droplet in the wettedWenzel and the nonwetted

CassieBaxter state [16].phobicity for superhydrophobic surfaces can be lost in cold and humidenvironments due to condensation and freezing [1,6,16]. Additionally,deterioration of the anti-icing properties of the superhydrophobic sur-faces was observed during cyclic icing and de-icing tests. The deteriora-tion was attributed to wearing of the surface. The surface asperitieswere gradually broken during icing and de-icing, due to volume expan-sion as thewater froze [1,16]. Such an observation is only possible, if theice had penetrated into the surface structure due to prior condensationand wetting. Thus it is necessary to further develop superhydrophobicsurfaces if they are to be used as anti-icing coatings. Both the durabilityof superhydrophobic coatings and the prevention ofwetting due to con-densation require further research.

To investigate frost formation on superhydrophobic surfaces in ahumid atmosphere, two water phase transition phenomena have tobe studied namely condensation and freezing. Condensation on super-hydrophobic surfaces can occur at surface temperatures below thedew point of water, and freezing below the melting point of water [8].Both processes can be described by nucleation and growth of a secondphase. Both condensation and freezing can be triggered by homoge-neous, or heterogeneous, nucleation if the primary phase is in contactwith a surface or with contamination. With respect to condensationand freezing the, two nucleatingmechanisms are discussed in more de-tail in the next section.

2. Nucleation of condensate and ice

Homogeneous nucleation in a supersaturated vapour phase, or asupercooled liquid, is initiated by the spontaneous assembly of amolec-ular cluster into a more stable liquid, or solid phase [3335]. During theformation of a nucleus, a new interface is formed and therefore the sur-face energy of the system increases. However, concomitantly, the freeenergy per transformed volume reduces, since the new phase is ener-getically more favourable. The total change in the free energy, due tothe phase transition can be described by [35]:

G surface termvolume term 4r2AB 4=3 r3GX 6

where G= change in the free energy (J), r = radius of nucleus (m);AB=interfacial energy between phase A and B (J/m2); GX=free en-ergy change per volume transformed (J/m3).

As the nucleus forms, the free energy of the system increases, until acritical nucleus size is reached. For larger nucleus sizes the energy of thesystem decreases, and the nucleus spontaneously grows. The evolutionof the free energy change during nucleation and its surface and volumeterm as a function of the nucleus radius are presented in Fig. 3, alongwith the critical nucleus radius and the critical activation energy fornucleation.

The critical nucleus size is dened as the radius r* at themaximumofthe free energy change. It can be calculated as following [35]:

d G =dr 8rAB4r2GX 0 7

r 2AB=GX: 8

To reach the critical nucleus size, the system has to overcome thecritical activation energy. By combining Eq. (8) with Eq. (6), the criticalactivation energy for the critical nucleus size is dened as:

G 16=3 AB3=GX2: 9

For homogeneous nucleation during the condensation of water va-pour into water droplets, the free energy change per transformed vol-ume can be described as a function of the supersaturation of vapour[34]:

G RT=v ln S 10

49nterface Science 210 (2014) 4757X

50 L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757where R= ideal gas constant (J.m3/mol K); T= absolute temperature(K); v=molar volume of molecule (m3/mol); S=supersaturation.

The critical activation energy for condensation therefore decreaseswith increasing supersaturation [34]:

G 16=3 AB3= RT=v ln S 2: 11

The free energy change per transformed volume, for homogeneousnucleation of ice into water can be described as a function ofsupercooling (T) of water [35]:

GX T=Tm Hf 12

whereT=difference between actual temperature of systemandmelt-ing temperature (K); Tm=melting temperature (K); Hf=latent heatof fusion (J/m3).

The critical activation energy for freezing decreases with increasingsupercooling and is given by [34]:

Fig. 3. Total change of the free energy G (solid) during nucleation, the surface term(dashed) and the volume term (dotted) as a function of the radius of the nucleus. The crit-ical radius (r) above which nuclei spontaneously grow is dened at the maximum totalfree energy of the system [35].G 16=3 AB3= T=Tm Hf 2: 13

For the extreme case of suppressed vibration and contamination,and under the condition of almost zero gravity, a maximum super-cooling of water of45 C at a pressure of 1 bar is expected [36]. Thistemperature corresponds to the assumed freezing temperature ofwater under homogeneous ice nucleation [37,38]. Despite this predic-tion, for the case of condensation and freezing on superhydrophobicsurfaces, heterogeneous nucleation at the surface is expected. Heteroge-neous nucleation can take place in a meta-stable phase which is in con-tact with the solid surface, or at contaminations. The critical activationenergy for nucleation is decreased by heterogeneous nucleation. It canbe calculated as following [35]:

Ghetero Ghomo 2 cos 1cos 2=4 14

where Ghetero = heterogeneous critical activation energy for nucle-ation (J); Ghomo=homogeneous critical activation energy for nucle-ation (J); =contact angle ().

The heterogeneous critical activation energy for the nucleation ofwater or ice decreases as the hydrophilicity of the surface increases,and the phase transitions occur with less effort. For hydrophobic sur-faces the critical activation energy for nucleation approaches that ofthe homogeneous critical activation energy for nucleation. Therefore,to provide the larger critical activation energy for nucleation, hydropho-bic surfaces require an increased supersaturation level for water vapourcondensation than for hydrophilic surfaces, and more supercooling ofthe condensed water droplets for freezing [35]. Maximising the contactangle is therefore the ideal option in order to delay nucleation of waterdroplets from supersaturated vapour and ice from supercooled water.

3. Freezing of droplets

After water vapour condensates into water droplets on the cooledsuperhydrophobic surface, formation of ice occurs at temperaturesbelow the freezing temperature of water [8]. Four stages can be distin-guished for the freezing of a water droplet on a surface, namely(i) water cooling, (ii) rapid kinetic freezing, (iii) isothermal freezing,and (iv) ice cooling [39]. Upon water cooling, the temperature of thewater droplet on the superhydrophobic surface initially decreasesuntil sufcient supercooling is provided for the nucleation of ice[35,39]. In the rst freezing stage, as the substrate is cooled, the mor-phology of the deposited water droplet changes. In Fig. 4 the dropletcontact angle is decreasing with a concomitant increase in the contactarea during cooling. One rationale behind this observation is the super-saturation of water vapour in the proximity of the three phase contactline modifying the solidvapour and solidliquid surface tensions. Con-sequently, the surface appears more hydrophilic at decreasing surfacetemperatures. The increase in interfacial area between the liquid drop-let and the surrounding air occurs despite the increasing surface tensionof water at reduced temperatures [40].

The heterogeneous nucleation of a water droplet, on a surface, isoften observed at the three-phase contact line (i.e. at the liquid, solidand vapour interface) [6]. During stage two of the freezing of a waterdroplet rapid kinetic crystal growth takes place from the initial pointof nucleation, progressing through thewhole droplet. As a consequencethe temperature increases due to the release of latent heat of fusion.Rapid kinetic crystal growth stops, when the equilibrium freezing tem-perature of water is reached at 0 C [39]. Rapid kinetic crystal growth isvisually observed by a change in the transparency of the water dropletfrom a clear to opaque state within a fraction of a second [7]. Fig. 5 pro-vides an example of this process.

Different explanations are given for this observation in the literature,with one possibility being that the droplet becomes opaque because ofthe release of air bubbles, since air is less soluble in ice than inwater. An-other explanation is light scattering due to the partially solidiedwater.The formationof a solid ice shell at the droplet surface, with a liquid cen-tre, has also been suggested [39].

Despite the change in optical transparency, the droplet is yet tocompletely transition to the solid state. The completion of this transitionfrom liquid to solid occurs in the third stage of the freezing process. Dur-ing the third freezing stage the temperature remains at the equilibriumfreezing temperature ofwater, namely 0C. A. freezing front is observed,which moves from the dropletsolid interface to the top of the droplet(Fig. 6). The droplet deforms at the top during freezing due to the differ-ent specic volumes of ice and water [7], irrespective of whether thesurface is hydrophobic or superhydrophobic [41].

The freezing rate of this isothermal freezing stage is mainly con-trolled by the rate of heat conduction to the substrate and dissipationto the environment by convection [5,35,39]. The heat transfer throughconduction between the interfaces of two materials is described bythe following equation [42]:

dQ=dt hA T2T1 15

where dQ/dt = heat transfer rate (J/s), h = heat transfer constant(J/m2 s K); A = interfacial area (m2); and T1 and T2 = the initial

and nal temperatures (K).

The result gives a strong indication that the freezing of droplets on asuperhydrophobic surface is linked to the condensation ofwater on that

Fig. 4. Change in the morphology of a droplet as a function of time during coo

51L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757The relationship given by Eq. (15) can be applied to the substratedroplet, substrateice and icewater interfaces during stage three offreezing of a water droplet. The heat transfer by means of conductionis linearly related to the interfacial area. Awater droplet of given volumehas a smaller interfacial area on a hydrophobic surface than on a hydro-philic surface due to its larger contact angle. As a consequence a lowerheat transfer rate is achieved with a larger contact angle, and freezingis delayed [22]. A water droplet residing in the CassieBaxter state ona superhydrophobic surface has an even lower heat transfer rate com-pared to one residing in the Wenzel state for two reasons. Firstly, thenon-wetting surface structure contains an air fraction with inherentinsulating properties. Therefore, the thermal barrier for heat transfer isincreased by the air layer in the CassieBaxter state [18]. Secondly, theeffective contact area between the substrate and liquid or substrateand ice is lower for the CassieBaxter state due to the lower solid frac-tion in contact with the droplet [7]. The lower interfacial area thusleads to a lower heat transfer rate [42]. A superhydrophobic surfacewhere the CassieBaxter state is retained during condensation, in theo-ry, is an ideal mechanism by which to reduce the heat transfer from thesurface to the condensed water droplets. Additionally, the velocity ofthe freezing front is also slowed down [7,22]. After freezing, the temper-ature of the frozen water droplet is further decreased in the last stageuntil the surrounding environment temperature is reached [39].

The freezing of droplets on a surface can typically be observed using atemperature controlled microscope stage. When neighbouring dropletsare monitored an interesting freezing phenomenon can be capturedthat gives insight into the freezing mechanism. For example, whenmacro-droplets of water are investigated on cooled superhydrophobicsurfaces, different freezing times for the droplets are typically recorded.In Fig. 7 an example experiment shows two neighbouring droplets resid-ing in close proximity to each other. Close observation reveals that theleft handdroplet undergoes freezing rst, which produces the character-istic distorted drop shape (for reasons described previously). What isalso simultaneously observed is a frost frontmoving across the substratesurface (as shownby the arrow). The frost front is created by the freezingof condensed water on the surface. As will be discussed later, this con-densed water already resides on the surface. Heterogeneous nucleationFig. 5. Change in transparency during rapid kinetic freezing of a droplet within a timeframe of less than 1 s.substrate. Recent observations by Boreyko and Collier [43] give furtherevidence of the critical role such a frost front plays in the freezing ofmacro-droplets. Effort should then be placed into understanding thecondensation, and subsequent freezing of water on superhydrophobicsurfaces to elucidate this phenomenon.

4. Condensation on superhydrophobic surfaces

If a superhydrophobic surface has a spatially uniform surface energy,condensation of water by heterogeneous nucleation in a supersaturatedsystem can occur both on top and between surface asperities [6]. Drop-let formation occurs through nucleation, growth and coalescence [28].Droplets smaller than the typical length scale of the surface roughness(i.e. micrometre scale) condense and grow in a manner similar to drop-lets on a planar hydrophobic surface [30], thereby wetting the surfaceasperities during condensation. The principle of Ostwald ripening canbe applied to the process of coalescence [34]. During Ostwald ripening,larger particles grow at the expense of smaller particles which shrinkand disappear. Since smaller particles have a larger surface area to vol-ume ratio, and therefore a larger surface energy per volume, the free en-of ice in the condensed water micro-droplets can occur at the edge of amacro-droplet or at sample defects (such as the edge of the superhydro-phobic coating) thereby, initiating the propagation of a frost front be-tween condensed macro-water droplets. Once the frost front contactsthe macro-droplet, seeding takes place and ice growth occurs throughthe macro-droplet. This process is called self-seeding, where ice formedin micro-droplets seed the growth of ice in macro-droplets. In this sce-nario no supercooling is required for the growth of ice on seed crystals[50], and no critical activation energy for nucleation is required for thefreezing of macro-droplets, as in the case for heterogeneous nucleationof ice [35]. Thus the observations here tend to indicate that the freezingdelay time associated with a macro-droplet is determined by self-seeding from ice in the propagating frost front, which occurs as a resultof condensed water micro-droplets on the surface.

ling from room temperature to10 C on a superhydrophobic coating.ergy of the system is reduced through a process known as Ostwaldripening. This effect leads to a reduction in the number of droplets ob-served on the surface and to a sharpening of their volume distribution(be it to larger droplet volumes) [35,44].

When condensed droplets are larger than the length scale of the sur-face roughness, their coalescence mechanism is inuenced by the sur-face asperities. Ostwald ripening can then take place between, or ontop of the surface asperities. Droplets residing on top of the superstruc-tures can be drawn onto the surface substructures by coalescence withlarger droplets in the surface cavities. Thereby resulting in a surface thatis in a wetting Wenzel state [30]. Alternatively the droplets can also betransported to the top of the superstructures by coalescence with drop-lets on top of the surface asperities. Thereby larger droplets form in thenon-wetting CassieBaxter state [30]. This is only possible if the rough-ness scale is small enough compared to the condensed water droplets.

Therefore, smaller roughness scales are favoured for retaining the non-wetting state of a superhydrophobic surface during condensation.

To investigate the condensation and freezing of water on super-hydrophobic surfaces, model pillar structured coatings were preparedfor this study. As a substrate for the pillar coatings, borosilica glass(Pyrex, HA Groiss & Co) is used. Before spin coating of the epoxy basedSU-8 photoresist (MicroChem Corporation), the borosilica glass sub-strates are rinsed with acetone and isopropyl alcohol (IPA), driedunder a nitrogen ow and treated in a plasma cleaner (Harrick Plasma)for 5 min with radio-frequency air plasma. Additionally, the borosilicaglass substrates are immersed into a 1M potassium hydroxide aqueoussolution (Chem-Supply) for 2min. Afterwards, the substrates are rinsed

withwater, cleanedwith acetone anddried under a nitrogen ow. Final-ly, the samples are immersed into a solution of (3-Glycidyloxypropyl)trimethoxysilane (3-GPMS, 5 wt.%, Aldrich) in ethanol. The substratesare rinsed with acetone and IPA and are dried under a nitrogen owfor 24h at room temperature.

Once the cleaning of the borosilica glass substrates is nished, the

Fig. 6.Movement of the icewater interface through a droplet during isothermal freezing of cooling from room temperature to10 C on a superhydrophobic coating.

52 L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757Fig. 7.Movement of a frost front across the substrate as a function of time upon cooling thesuperhydrophobic substrate from room temperature to10C. (c) Upon contactwith thefrost front, seeding of ice in the water droplet on the right is observed by the change inoptical properties of the droplet.SU-8 photoresist is spin coated onto the substrateswith a spin speed be-tween4700 and1300rpm to achieve coating thicknesses between 5 and25m. Afterwards, the coatings are soft baked for 2 to 3min at 65C andfor 5 to 7min at 95C. The samples are thenmaskedwith the pattern tocreate the pillar structures, and exposed to UV-light with an intensity of250 to 400mJ/cm2. The mask is then removed, and samples are post-exposure baked for 1min at 65 C and then 4min at 95 C. Finally, thesamples are developed in SU-8 developer (MicroChem Corporation)for 1 to 5 min, rinsed with IPA to remove unreacted SU-8 from themasked regions, and dried with compressed air. In a last step the pillarcoatings are hard baked for 1min at each 65 C and 95 C, for 5min at150 C and again for 1min at each 95C and 65C.

The pillar parameters are presented in Fig. 8 including the pillardiameter (D), height (H) and distance (L) and the angle between thelattice vectors (). is kept at 60 for all types of pillar coatings.

Three different pillar geometries are utilized in this study for obser-vation of the condensation and freezing mechanism. Table 1 outlinesthe geometries for the samples, with the main variations between thethree being (i) changing gap between pillars for a xed solid fraction,and (ii) changing solid fraction for a xed gap between pillars.

To increase thehydrophobicity of the samples, a 15nmcoating of thehydrophobic SH-HT (SHHT, DON Co.) material is added by electronbeam evaporation. The adhesion of the hydrophobic coating to the pillarstructure is enhanced by an argon plasma treatment of the samples for30 s prior to the deposition of the SH-HT material.Fig. 8. Schematic illustration of the pillar parameters, including the pillar diameter(D), distance (L), height (H), gap width (w) between the pillars and angle between thelattice vectors ().

These pillar structures were subjected to condensation and freezing,and investigated in-situ by optical microscopy. The condensation andfreezing of water droplets on the different pillar coatings is discussedwith respect to how the frozen condensate impacts on the freezing oflarge droplets of water on the same surface. Microscopy images duringwater condensation on the pillar coatings with; the smallest pillar di-mensions, the largest pillar dimensions and the largest solid fractionare presented in Fig. 9. As shown, condensation of micro-droplets canstart between the pillars for all of the investigated coatings. However,for the pillar coating having the smallest pillar dimension (Fig. 9ac),the condensed water micro-droplets quickly (within a minutetimeframe) overgrow the pillar structure.

Spherical water micro-droplets are formed tominimise their surfacearea and energy. They build up above the pillar structure and reside on aCassieBaxter state surface. Water micro-droplets on the pillar coatingwith the largest pillar dimensions mainly condense and growing be-tween thepillar structures, as observed in Fig. 9df. For this type of pillarcoating the roughness scale is too large to have an inuence on the in-terfacial area between the water droplet and the surface. Therefore,the water micro-droplets only experience the side walls of the pillarsand the bottom surface of the structure. Spherical water micro-droplets are formed between and around the pillars to minimise theirsurface energy. This pillar coating,with the largest solid fraction and pil-lar dimensions, maintains a wettingWenzel state during condensation.Condensedwatermicro-droplets on the pillar coating having the largestsolid fraction are growing both between and on top of the pillar struc-ture, as observed in Fig. 9gi. The roughness scale is small enough to in-uence the contact area between the water micro-droplets and thesurface, however, the individual pillars are still inuencing the shapeof thewatermicro-droplets. This sample, with the largest solid fraction,is in a wetting Wenzel state during condensation.

Microscopy images during coalescence of condensed water micro-droplets on the pillar coatings with the smallest pillar dimensions(Fig. 10a and b), the largest pillar dimensions (Fig. 10c and d) and thelargest solid area fraction (Fig. 10e and f) are presented. Condensedwater micro-droplets on the substrate with the smallest pillar dimen-sions are coalescing on top of the pillars, while remaining temporarily

Table 1Design of the rst pillar serieswith increasing pillar dimensions, including D, H, L, w andS.

Sample Pillardiameter (D)

Pillarheight (H)

Pillardistance (L)

Gapwidth (w)

Solidfraction (S)

[m] [m] [m] [m] [%]

Small pillarsubstrate

2 5 5 3 13

Large pillarsubstrate

10 25 25 15 13

Large solidfractionsubstrate

8 5 11 3 42

53L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757Fig. 9. Condensation and growth of micro-droplets on a pillar coating: Small Pillar Substrate wia) 13.4 C, b) 1.1 C and c) 13.4 C. Large Pillar Substrate with a pillar diameter, height and distaLarge Area Fraction Substrate with a pillar diameter, height and distance of 8, 5 and 11 m at ath a pillar diameter, height and distance of 2, 5 and 5 m at a cooling stage temperature ofnce of 10, 25 and 25mat a cooling stage temperature of d) 29.6 C, e) 1.4 C and f) 20C.

cooling stage temperature of g) 28.4 C, h) 20.1 C for 0 s and i) 20.0 C for 105 s.

pinned to the original location. Therefore, this pillar coating can beinterpreted to be at least partly wetting with the droplet contact line(partly) penetrating below the pillars for both the condensed and coa-lesced micro-droplets.

Condensedmicro-droplets on the substratewith the largest pillar di-mensions are coalescing at the base of the pillar structure, as presentedin Fig. 10c and d. The coalescedmicro-droplets are still smaller than thepillar separation distances and experience the hydrophobic bottom sur-face and side walls of the pillars. As a result the pillar structure remainswetted, an indication that this, rather than wetting the substrate bot-tom, produces the lowest energy state. Condensed micro-droplets onthe substratewith the largest solid area fraction are coalescing betweenand on top of the pillar structures as presented in Fig. 10e and f. The co-alescedmicro-droplets are temporarily pinned to their original positionand the pillar structures remain wetted. Spherical micro-droplets startto form on top of the surface coating, to minimise their surface areaand energy. Those micro-droplets are in a wetted Wenzel state.

When the freezing mechanism of ice for the condensed micro-droplets is investigated, an interesting mechanism is observed. Micro-scopic images of the freezing process for the substrates with thesmallest pillar structure, largest pillar structure and largest solid fractionare presented in Fig. 11. Dendritic ice crystals grow outwardly from theperimeter of the frozen water droplets, as observed for the substratewith the smallest pillar dimensions (Fig. 11ab). Such crystal growthhas been observed before on similar superhydrophobic surfaces [43].

54 L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757Fig. 10. Coalescence of condensedmicro-droplets on a pillar coating: Small Pillar Substrateat a cooling stage temperature of20.0 C at a) 0 s and b) 15 s. Large Pillar Substrate at acooling stage temperature of20.0C for c) 0s and d) 5s. Large Area Fraction Substrate at

a cooling stage temperature of20.0 C for e) 190 s and f) 195 s.Dendritic ice crystals are observed to seed ice in neighbouring watermicro-droplets upon contact. The presence of a water aerosol in thehumid surroundings was recently shown not to seed ice nucleation indroplets [45], thus conrming the dendritic crystals are responsible.Self-seeding of ice from dendritic ice crystals is therefore responsiblefor the spreading of a frost front between condensed micro-droplets.Guadarrama-Cetina et al. describe such behaviour as percolation-induced frost formation [46]. For the substrate with the smallest pillardimensions, dendritic crystals have to grow on top of the pillars toreach neighbouring micro-droplets. Since the micro-droplets are muchlarger than the roughness of the surface, thosemicro-droplets are isolat-ed by the pillar structures. Hence, these two factors may help delay thespreading of a frost front between condensed micro-droplets on thissubstrate.

Frozen micro-droplets, on the substrate with the largest pillar di-mensions form dendritic ice crystals as well. Dendritic ice crystalsbetween the pillar structures seed ice in neighbouring droplets. Den-dritic ice crystals grow on top of the pillar structures, after ice seedingby neighbouring micro-droplets has occurred. Micro-droplets on thesubstrate with the largest pillar dimensions are too small to be isolatedby the pillar structures, and only experience the hydrophobic surfacebottom and side walls of the pillars. The shape of the micro-droplets isnot always spherical on this substrate but contort around the pillarstructures, which is an indication of pinning potentials. The distance tothe nearest neighbours for such deformedmicro-droplets can thereforebe smaller, than for spherical droplets. Due to these factors the spread-ing of a frost front between condensed droplets is expected to be fasteron the substratewith the largest pillar dimensions compared to the sub-strate with the smallest pillar dimensions.

For the substratewith the largest solid fraction and pillar dimensionsthe formation of dendritic ice crystals from frozen micro-droplets is ob-served (Fig. 11fh). Dendritic ice crystals between the pillar structuresare responsible for the seeding of ice in neighbouring droplets, unlikethe substrate with the smallest pillar dimensions. This substrate, withthe largest solid fraction, has the same gap width between the pillarsas the substrate with the smallest pillar dimensions, but a much lowerpillar density. Dendritic ice crystals grow on top of the pillars after iceseeding by neighbouring micro-droplets has occurred. The dendriticice crystals are inuencedmore by the pillar structures with the largestsolid fraction than by the pillar structures with the largest pillardimensions.

In summary, all the investigated substrates produced dendritic icecrystal growth which initially extended from the frozen water droplets.Upon contact of those dendritic ice crystals with neighbouring droplets,ice seeding and growth occurs, thereby producing a freezing frontwhich spreads over the substrate surface. When compared to the sizeof the condensate droplets, if the pillar separation distance is smallenough and pillar density large enough, the droplets themselves are in-uenced by the pillar structure. As a result, seeding of ice in neighbouringdroplets by dendritic ice crystals is also inuenced by the underlying pil-lar structure. The spreading of a freezing front is inhibited on suchsamples and frost formation is delayed, but not inhibited. To show thatin the limit of very small roughness the front still occurs, this freezingfront phenomenon was conrmed on a smooth hydrophobic coating(Fig. 12) using the same hydrophobic thin lm material, but withoutthe nanolithographic step used to create the pillar structures.

It must be acknowledged here that the specic conditions of theseexperiments (such as humidity, air temperature and substrate temper-ature)will inuence themagnitude of the response observed. However,this does not impact on the comparative trends observed for the differ-ent pillar structures. The fact that despite the high water contact angleon these surfaces there is condensation and freezing of condensate onthe surface indicates that such surfaces for anti-icing applications stillrequiremuchdevelopment. A broader range of samples havingdifferentpillar geometries have been included in a further analysis where the

freezing delay time has been measured for condensation and for

Fig. 11. Self-seeding and freezing of condensedmicro-droplets on a pillar coating: Small Pillar Substrate at a cooling stage temperature of20.0 C for a) 310 s and b) 320 s. Large Pillar Sub-strate at a cooling stage temperature of20.0 C for c) 150s, d) 165s and e) 180 s. Large Area Fraction Substrate at a cooling stage temperature of20.0 C for f) 190s, g) 195s and h) 275s.

Fig. 12. Formation and growth of ice crystals extending from frozen droplets of condensedwater on a smooth hydrophobic surface presented as a function of time from a) through f). Notethat the subsequent freezing of the neighbouring droplets occurs once the growing ice crystals make intimate contact.

55L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757

macro-droplets. This comparison is provided in Fig. 13,where thediffer-ent geometries have been characterised by the rms roughness of thesamples as determined using optical prolometry (WykoNT9100,Veeco) over several mm2 area. The rms roughness from prolometryis provided as a simple measure to differentiate the samples, and byno means truly captures the geometric differences between the sam-ples. Surprisingly the freezing delay times for the condensate andmacro-droplets are of a similar time scale. This is despite the condensate(micro-droplets) being approximately 106 times smaller volume thanthe macro-droplets. If the freezing of any size droplet is dependent onheat ow out of the droplet volume, the micro-droplets would havebeen expected to freeze much faster.

From these experimental results one can conclude that heat transferplays aminor role on the freezing delay time and that freezing inmacro-droplets is not initiated by heterogeneous nucleation at the interfacebetween the solid and water. Instead, ice formation and growth inmacro-droplets is triggered by ice crystal seeds. From the collection ofmicroscopic images (Fig. 11) presented, dendritic ice crystals growfrom the frozen droplet perimeter thereby seeding ice in neighbouringdroplets. It is in this way that a freezing front propagates between con-densedwatermicro-droplets. Themovement of a freezing front also ini-tiates freezing in macro-droplets, as illustrated in Fig. 7. The spreadingmechanism of a freezing front determines the measured freezingdelay times of both micro- and macro-droplets. However, the originalnucleation mechanism for the condensed water micro-droplets needs

Superhydrophobicity has been widely reported as a desirable prop-

subsequent freezing of the condensate, that plays a key role in macro-scopic ice formation. Experimental studies have shown that a freezingfront develops and moves across the superhydrophobic surface, whichin turn acts as the seed to induce freezing of attached water droplets(micro- and macro-) upon intimate contact. The superhydrophobicityacts to slow the rate of water condensation and freezing of said conden-sate, but does not eliminate it completely.

It is concluded, that rapid kinetic freezing of water macro-droplets isinitiated by self-seeding of ice from a freezing front that develops be-tween condensed water micro-droplets. Under an optical microscope itis readily observed that dendritic ice crystals grow on the perimeter offrozen water droplets. Those dendritic ice crystals seed ice formation inthe neighbouring water droplets upon contact. The nucleation mecha-nismof ice in the condensedwatermicro-droplets therefore has an inu-ence on the freezing delay time, and should be the subject of futureexperimental investigations to elucidate the phenomenon. Visually, itwas observed that heterogeneous nucleation of ice in condensed watermicro-droplets occurs at the edge of the substrate. Therefore, the nucle-ation mechanism of ice is expected to be similar for all types of sampleswith the propagation rate of a freezing front originating at the substrateedge being themain factor inuencing the freezing delay time formacro-droplets. Thus attempts to signicantly inhibit the freezing of water on asurfacemay need to call uponmore than just the superhydrophobicity ofthe surface to achieve this desired result. Surfaces that retard the forma-tion of water condensate are equally as important, as this in turn will re-

ater

56 L. Oberli et al. / Advances in Colloid and Interface Science 210 (2014) 4757erty for a coating to possess when considering applications that areprone to degraded performance or damage arising from ice formation.Despite these coatings' ability to limit or retard the attachment ofwater droplets to their surface, such coatings do not exclude the abilityfor water to condense on the surface. It is this condensation, and the

Fig. 13. Freezing delay times of both pillar series as a function of the pillar distance for thewto be further investigated and this is the subject of further experimentalwork. Visual observation indicates that heterogeneous nucleation oc-curs in condensed water micro-droplets at the edge of the substrate.Therefore, thepropagation of a freezing front ismost likely themain fac-tor inuencing the measured freezing delay times.

5. Conclusions and outlookpillar geometries have been characterised by the rms roughness as determined by optical produce dendritic ice crystal growth. These ice crystals are experimentallyobserved as a freezing front that acts as seeds to induce the formationof ice within liquid water droplets. Future investigations should focuson the dynamic processes of watersurface interactions; not dynamiccontact angle studies per se but on condensation and freezing of water.

For real life applications such as externallymounted car sidemirrors,condensation at low temperatures cannot be easily avoided. Inhibitingnucleation of ice in condensed water droplets becomes the key factorin preventing ice formation. Such nucleation will always start at someform of defect on the surface (be it structural, chemical or contamina-tion), such as the edge of the anti-icing coating. To achieve retardationof frost formation the said coatings must be defect free; the challengetherein lies with creating superhydrophobic coatings that are robustenough to withstand environmental exposure, over large areas (fromcm2 to m2 in area).

droplet freezing experiment and the condensation and freezing experiment. The different

lometry.

Acknowledgements

This original research was proudly supported by SMR-Technologiesand the Commonwealth of Australia, through the Automotive Australia2020 Cooperative Research Centre (AA2020CRC). The authors thankthe Australian National Fabrication Facility SA Node for preparation ofthe superhydrophobic surfaces.

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Condensation and freezing of droplets on superhydrophobic surfaces1. Introduction2. Nucleation of condensate and ice3. Freezing of droplets4. Condensation on superhydrophobic surfaces5. Conclusions and outlookAcknowledgementsReferences