International Journal of Multiphase Flow 62 (2014) 30–36

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International Journal of Multiphase Flow

journal homepage: www.elsevier .com/locate / i jmulflow

Acoustic characteristics of fluid interface displacement in drying porousmedia

http://dx.doi.org/10.1016/j.ijmultiphaseflow.2014.01.0110301-9322/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: School of Chemical Engineering and AnalyticalScience, The University of Manchester, Sackville Street, Manchester M13 9PL, UK.Tel.: +44 161 3063980.

E-mail address: nima.shokri@manchester.ac.uk (N. Shokri).

N. Grapsas a, N. Shokri b,⇑a Department of Earth and Environment, Boston University, Boston, MA, USAb School of Chemical Engineering and Analytical Science, University of Manchester, Manchester, UK

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

Article history:Received 15 November 2013Received in revised form 28 January 2014Accepted 30 January 2014Available online 10 February 2014

Keywords:Fluid interface displacementEvaporationPorous mediaParticle sizeAcoustic emission

Water evaporation from porous media involves many rapid interfacial jumps at the pore-scale as airinvades the pore network and displaces the evaporating fluid. We show that this process produces acrackling noise that can be detected using an acoustic emission (AE) instrument. We investigated theacoustic signature of evaporation from porous media using transparent glass cells packed with five typesof sand and glass beads differing in particle size distribution and grains shape. Each sample was mountedon a digital balance, saturated with dyed water, left to evaporate under well-controlled atmospheric con-ditions, and digitally imaged every 20 min to quantify the dynamics of liquid phase distributions. An AEsensor was fixed to each column to record AE events (hits) and their acoustic features. Results indicatethat the cumulative number of AE hits is strongly proportional to total evaporative losses. Additionally,the cumulative number of hits shares an inverse relationship with particle size and a direct relationshipwith grain irregularity. Analysis of the dynamics of liquid phase distributions reveals a strong correlationbetween the area invaded by air and the cumulative number of AE hits. Our results suggest that AEtechniques may hold the potential to non-invasively analyze evaporation from porous media.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Evaporation from porous media is a process ubiquitous in manyindustrial and environmental contexts ranging from the drying ofcatalysts, paper, pharmaceutical products, and porous buildingmaterials to water evaporation from soil which influences thehydrological cycle and various biological activities in the vadosezone. Evaporation from saturated porous media typically involvesthe invasion of pores by a non-wetting gaseous phase (e.g. air) thatdisplaces a resident wetting liquid phase (e.g. water) resulting inthe formation of a receding primary drying front, defined as theinterface between saturated and partially saturated zones(Shaw, 1987; Shokri et al., 2012).

Many studies have investigated the drying of porous materialsunder various boundary conditions and have analyzed the effectsof parameters such as atmospheric conditions, wettability, porousmedia transport properties, and the physical and chemical proper-ties of evaporating fluids (Scherer, 1990; Laurindo and Prat, 1998;Yiotis et al., 2006; Chapuis and Prat, 2007; Lehmann et al., 2008;

Faure and Coussot, 2010; Shokri and Salvucci, 2011; Peyssonet al., 2011; Shahraeeni and Or, 2010; Smits et al., 2012; Shokriand Sahimi, 2012; Doel et al., 2012; Shokri et al., 2012; Yiotiset al., 2012a,b; Aminzadeh and Or, 2013; Haghighi et al., 2013;Shokri and Or, 2013; Or et al., 2013). When evaporation fromsaturated porous media initiates, evaporation rates are mainly con-trolled by atmospheric demand, are rather high, and remain rela-tively constant – these features characterize what is referred toas stage-1 evaporation (Saravanapavan and Salvucci, 2000; Shokriand Or, 2011). During stage-1, capillary-induced liquid flow trans-ports the liquid from a receding primary drying front (referred tosimply as ‘drying front’ from here on) to the surface where watervaporization takes place. When the drying front recedes to a char-acteristic depth which can be estimated using the capillary pres-sure–saturation curve of the medium (Lehmann et al., 2008;Shokri and Salvucci, 2011), hydraulic connectivity with the surfacedisrupts, resulting in a transition to lower evaporative fluxes thatmarks the onset of stage-2 evaporation. During this period, liquidvaporization occurs inside the porous medium and evaporationbecomes limited by vapor diffusion through the overlying dry layernear the surface (Scherer, 1990; Shokri and Or, 2011).

It is well-known that air invasion of saturated porous media pro-duces many discrete, rapid pore-scale interfacial bursts known as‘‘Haines jumps’’ which result from instabilities at the contact lines

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of menisci as pores are drained (Haines, 1930; Lu et al., 1994; Bluntand Scher, 1995; Aker et al., 2000; DiCarlo et al., 2003; Chotard et al.,2006; Crandall et al., 2009; Moebius et al., 2012). Recent studieshave shown that these pore-scale events may generate soundsdepending on the process under consideration. For example, DiCarloet al. (2003) found that drainage from porous media generates anacoustic ‘‘crackling noise’’ composed of many individual acousticevents. Using passive acoustic emission (AE) methods oftenemployed to monitor the accumulation of strain in civil structuresand in microseismicity studies (Nair and Cai, 2010), each smallevent was detected as a ‘‘hit’’ and demonstrated to be related tothe displacement of the liquid–gas meniscus in porous media.Chotard et al. (2006) reported that these AEs could also be detectedduring cement drying and that the cumulative number of hits maybe related to evaporative mass loss. Further inquiries undertakenrecently by Moebius et al. (2012) have revealed that AE hits gener-ated during fluid front displacement in porous media exhibit adependency on the medium pore size among other factors.

Motivated by the widespread importance of drying in porousmedia and recent advances in the application of AE techniques withregard to transport phenomena in porous media (Sahimi, 2011), thisstudy aims to evaluate the feasibility of applying non-invasive AEtechniques to characterize the drying of porous media. Within thiscontext, we present experiments which demonstrate that AEsgenerated during evaporation are related to Haines jumps and areindeed strongly linked to evaporative mass loss during evaporationfrom porous media. Additionally, we show that medium particlesize and grain surface irregularity also influence the acoustic signa-ture of the evaporation process which will be discussed in detail.

2. Materials and methods

The model porous media used in this work consisted of either sil-ica sand or glass beads across a range of particle sizes. Fig. 1 presentsthe particle size distributions (PSDs) of the media obtained usingCAMSIZER Digital Image Processing Particle Size from HORIBA. Thesand grains had average particle sizes (diameters, d) of 0.34 mm,0.58 mm, and 0.89 mm, and the glass beads had average particlesizes of 0.15 mm and 0.53 mm. These particles were selected so thatthe influence exerted on AE characteristics by changes in particlesize and roughness, and by extension pore space geometry, couldbe assessed.

As illustrated in Fig. 1a, the sand sample with an average parti-cle size of 0.58 mm possessed a particle size distribution quite

Fig. 1. (a) Particle size distributions of different sand and glass beads used in the experimedium in each cell. S and GB refer to sand and glass beads, respectively. (b) Schematic ofmounted on digital balances connected to a PC to record the dynamics of water evaporatiorecorded using a digital camera. An AE sensor was mounted to the rear of each cell usingAE machine with its corresponding features.

similar to the glass bead sample with an average particle size of0.53 mm. This provided the opportunity to constrain the effectsof grain shape and pore roughness on drying behavior since thesand grains were rough and shaped irregularly whereas the glassbeads were relatively smooth spheres.

Particles were saturated with a dye-water solution and packeduniformly into 8 cm � 8 cm � 0.5 cm transparent glass Hele–Shawcells closed on all sides except the top, which was left uncovered toenable evaporation. An identical procedure was used to pack par-ticles into each cell. 5 mL of blue dye was added to 1 L of waterand stirred inside of a Tupperware container. Importantly, usingdye increased the contrast between the phases present duringdrying making image segmentation possible. Dry particles weretransferred into this solution and mixed to purge trapped air andguarantee complete, uniform saturation. After saturating theparticles, some excess ponding solution was transferred into aHele–Shaw cell to a depth of �1 cm while ensuring that the parti-cles remained submerged and saturated. Using an indented spatulawith an effective diameter of 1/3 cm, small amounts of saturatedparticles were scooped from the Tupperware and transferred intothe fluid within the Hele–Shaw cell. Particles were transferred inthis manner until they accumulated to a height of �1.5 cm. At thispoint, a stir was used to gently mix the particles which were thenallowed to settle. This procedure of transferring, mixing, and com-pacting particles was repeated until particles accumulated to 6 cmwithin each Hele–Shaw cell. This process removed entrapped airbubbles and guarded against the layering effects that result fromtransferring particles in discrete increments. Managing layeringeffects is essential to fluid flow or evaporation experimentationas the incremental addition of particles forms distinct strata withindependent hydraulic properties.

An opaque white plastic backing with a cutout for an AE sensorwas taped to the rear of each cell to provide a consistent, uniformlycolored backdrop which aided during image segmentation of thetransparent glass bead samples. The cells were left to evaporateon digital balances (accurate to 0.01 g) in an environmental cham-ber set to 35 �C and 30% relative humidity. Mass measurementswere recorded every 5 min, and photos of the glass cells weretaken every 20 min with a computer-operated digital cameraintended to track changes in liquid phase distributions with aspatial resolution of 110 lm.

A model R6I-AST Physical Acoustic Corporation resonant AEsensor (45 kHz peak frequency and operating range from �23 dBto 117 dB) with a built in preamplifier was fixed to the rear of each

ments. The numbers in the legends indicate the average particle size of the porousthe experimental setup. Hele–Shaw cells packed with well-defined particle sizes aren while changes in liquid phase distribution during evaporation were automatically

duct tape. (c) Conceptual sketch of a typical acoustic emission signal recorded by the

32 N. Grapsas, N. Shokri / International Journal of Multiphase Flow 62 (2014) 30–36

Hele-Shaw cell on Dow Corning 7 Release Compound and linked toa Physical Acoustics Corporation Micro-II Digital AE system. Thissetup allowed AE waveforms to be recorded in units of mV andreports individual AE hit features such as absolute energy (hereafterreferred to as energy), amplitude, duration, and frequency inaddition to tallying the cumulative number of observed hits. Anamplitude threshold of 26 (dB) was used to remove backgroundnoise. Fig. 1b and c illustrate the experimental setup and theschematic of an AE waveform with the corresponding features.

3. Results and discussion

3.1. Sources of AE hits generated during evaporation from porousmedia

Moebius et al. (2012) have identified five AE-generating mech-anisms associated with fluid front displacement in porous media:Haines jumps, liquid bridge rupture, air bubble entrainment andoscillation, interfacial snap-off, and grain collisions. Each sourcemechanism gives rise to mechanical waves in a unique way. Hainesjumps likely release mechanical energy when menisci reduce theirinterfacial energies during jumps from less stable configurationsacross larger pore throats to more stable configurations acrosssmaller ones (Quere, 1997). Suspended liquid bridges, which areentrapped saturated regions left behind by the receding dryingfronts, rupture when evaporation from their surfaces causes themto shrink until they reach a critical volume (Orr et al., 1975). Themagnitude of interfacial energy reductions that occur during theseevents have been shown to be of similar order to those duringHaines jumps, but bridge ruptures are substantially rarer events(Simons et al., 1994). Air bubble entrapment, coalescence andoscillations, which occur when contact lines and fluid displace-ment fronts reconfigure while abruptly moving across rough sur-faces, emit sound either when air becomes entrapped from a freesurface or when bubbles snap-off from larger parent gas bodies(Manasseh et al., 2008). Other interfacial snap-off processes(Joekar-Niasar et al., 2008) have been studied comprehensively(Kovscek and Radke, 1996; Gauglitz and Radke, 1989, 1990) andare associated with the release of interfacial energy (Ransohoffet al., 1987). Mechanical collisions of the type that occur betweengrains during settling or in response to capillary forces andpressure waves have also been identified as one of a host ofpotential AE sources (Michlmayr et al., 2012). These five phenomenarepresent the processes which may contribute to the AE signaturesdetected during our evaporation experiments.

Fig. 2. (a–d) The cumulative evaporative losses and cumulative AE hits versus time meglass beads (GB).

We analyzed the time evolution of the number of AE hitsobserved, their amplitudes and energies generated duringevaporation from porous media. These observables constitute thecommonly used AE analytical features. Given that the peak45 kHz resonant frequency of our sensors corresponds to soundwavelengths in silica of 132 mm which is orders of magnitudelarger than the particle diameters in this study, AEs likely undergolittle scattering and propagate uniformly (Jia, 2004). The effects ofsignal attenuation on these observables were ignored due to thesmall volume of packed sand through which elastic waves had topropagate to reach an AE sensor.

3.2. AE hits and evaporative mass loss

When drying initiated, a receding drying front developed ineach medium as air began to invade and displace the evaporatingwater.

Fig. 2 presents typical curves illustrating the time evolutions ofevaporative mass loss and AE hits generated during evaporation.The observed mass loss pattern dynamics were consistent with datapreviously reported in literature (Shokri and Or, 2011). High constantdrying rates – indicated by the constant slopes of the mass loss curves– are initially present during stage-1 evaporation wherein capillaritydraws fluids from the saturated zone to the medium’s surface toevaporate. Discernible changes in drying rates occurred either whenthe saturated zones became depleted of fluid (which was the case forthe sand sample with an average particle size of 0.34 mm) or when asample entered stage-2 evaporation (as was the case for the sandsamples with average particle sizes of 0.58 mm and 0.89 mm andfor the glass bead sample with an average particle size of 0.53 mm)when the upward capillary force could no longer overcome thedownward gravitational force (viscous forces are negligible in ourexperiments due to the relatively large particle sizes).

As illustrated in Fig. 2, the AE hit rates are found to strongly cor-relate with mass loss rates in all cases, suggesting a direct relation-ship between AE hits and mass loss that was first observed byChotard et al. (2006). Before air began penetrating the pore spaces,the AE hit rates were zero indicating that background noise wasnot being captured and that the processes by which AEs are pro-duced had not yet commenced. Once air began invading the porousmedia, stage-1 evaporation started and the highest hit rates wereobserved. Since the drying rate is faster during stage-1, morepore-scale interfacial jumps are induced causing more AEs to beemitted. After the saturated regime was either exhausted or theprocess entered stage-2 evaporation, the AE hit rates dropped

asured using digital balances and AE sensors during evaporation from sand (S) and

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precipitously but maintained proportionality with mass loss. It isimportant to note that AE hits were only observed while evapora-tion was active and air invaded the medium, suggesting that AEsdominantly emerged from motions of the air–water interface. Thispattern was present for all particle sizes and roughnesses. Fig. 2shows the great potential of the AE technique to monitor, non-invasively quantify, and analyze the drying of a porous medium.It also illustrates the significant influence of particle size and grainshape (pore geometry) on drying behavior and on the emergentcharacteristics of AEs which will be discussed in following sections.

3.3. Linking AE hits to fluid invasion patterns

The dynamics of liquid phase distributions during evaporationwere quantified by segmenting the recorded images into binary datarepresenting the saturated and unsaturated phases following theprocedure described in Shokri et al. (2012). Fig. 3 depicts a represen-tative time progression of air invasion and drying front propagation.The white, red, green, and blue regions correspond to air-invadedareas after 5, 10, 15, and 25 h from the onset of evaporation andthe black region corresponds to the saturated region after 25 h.

We have calculated the time evolution of invaded area for allparticle sizes using the segmented images and compared the

Fig. 3. Typical air invasion patterns at macro-scale during evaporation from sandwith an average particle size of 0.58 mm. White, red, green, and blue indicate theinvaded region after 5, 10, 15 and 25 h respectively. Black corresponds to thesaturated region after 25 h.

Fig. 4. (a–d) Area of the invaded regions (IA) versus time in glass cells packed with sanddelineated by image analysis and compared to the recorded number of AE hits.

results to the recorded AE hits. Fig. 4 illustrates that a directproportionality is shared between the time evolution of AE hitsand invaded area (denoted as IA in the figures) during stage-1evaporation for all particle classes. Any region invaded by air expe-rienced many pore-scale interfacial motions. It follows that anychanges in invaded area necessarily stemmed from these pore-scale interfacial bursts, implying that the number of AE hitsrecorded directly corresponded to the number of Haines jumpsthat occurred in the medium.

In Fig. 4c, for sand with an average particle size of 0.89 mm, twolarge steps occur in invaded area approximately half a day into thetrial. Inspection of both the raw and binary images recorded atthose times reveals massive, rapid invasion events in the area ana-lyzed. These dramatic events can be explained by avalanches ofpore invasions which only stop once all the menisci involved stabi-lize across pore throats smaller than those they originally spanned.Such drastic invasion events were observed only during drying ofthe largest-diameter particle class due to its volumetrically largerpores. The presence of larger pores reduces the likelihood that apropagating meniscus will attain a stable configuration in a givenvolume after destabilizing – avalanches also occurred in smallerparticle classes but over smaller volumes and areas. The AE hit ratelikely remained constant in spite of these avalanches because ofthe limited image section that was used to assess invaded area;decreased hit rates from regions outside of the analyzed field ofview could have compensated for increased hit rates associatedwith the avalanches. The time resolution of our analysis may alsoexaggerate the impact of such avalanche events.

3.4. Influence of particle size on the AEs generated during evaporation

Hydraulic conductivity, porosity, and in particular the poregeometry of a porous medium are dependent on the constituentgrains’ sizes and shapes. Since AEs are generated by meniscusmotions in pores, pore-scale configuration may imprint a signatureon the crackling sound detected using AE equipment. In the follow-ing section, we discuss the effects of particle size and surfaceirregularity on the characteristics of resulting AEs.

3.4.1. AE events as influenced by the medium textureThe precise proportionality between AE hits and mass loss in

Fig. 2 as well as between AE hits and invaded area in Fig. 4 variesamong particle classes. This varying proportionality suggests that

(S) and glass beads (GB) with different average particle sizes. The invaded area was

Fig. 5. The cumulative number of AE hits observed as a function of the cumulativemass loss for each particle. S and GB refer to sand and glass beads, respectively.

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a medium’s particle size, surface roughness, and typical grain (pore)shape influence the characteristics of AEs emitted during evapora-tion from porous media. This can be rationalized since both poreshape and size influence the pore-scale liquid phase reconfigurationdynamics that ultimately mediate emitted AE signals. Fig. 5 displaysthe number of AE hits observed as a function of mass loss across allof the particles used in our experiments. This figure shows that hitrate varies indirectly with particle size.

Assuming a constant porosity of �0.4 in all cases, a mediumconsisting of larger particles has larger pores meaning that fewerpores are vacated per given volume of evaporated water. Therefore,when fluid evaporation evacuates an equal volume from two med-ia with distinct particle sizes, more interfacial jumps occur in thefiner medium leading to more AE hits. Fig. 5 also suggests thatmore mass is lost per hit in a coarser-textured medium due to lar-ger pore size, a result consistent with the aforementioned logic. Insummary, the total number of hits observed during evaporationfrom porous media is inversely related to pore size.

Pore geometry modifies the dynamics of capillary flow and fluiddisplacement during evaporation and should thus influence thecharacteristics of the resulting AEs. To understand the impactexerted by pore geometry, we compare the AE trends exhibited bysand with an average particle size of 0.58 mm to those yielded byglass beads with an average particle size of 0.53 mm. As depictedin Fig. 1, these two samples shared similar average particle sizesand distributions but differed in surface roughness and typical grainshape; the glass beads were smooth spherical particles whereas thesand grains were shaped irregularly. More AEs were detected perunit mass loss in the sand sample than in the similarly-sized glassbead sample. Intuitively, this result makes sense given the jaggednature of pores between rough grains of sand. The small protrusionsalong their surfaces provide extra locations for a destabilizedmeniscus to re-adhere, decreasing the displacement of a typicalHaines jump but increasing the occurrence frequency for a given

Fig. 6. The amplitude distributions of AEs observed during evaporation for each particlevalues of �0.11, �0.07, and �0.06 and for (b) GB 0.15 mm and GB 0.53 mm by �0.11 andsizes presented in the legend.

volume. Individual displacements would be of smaller volumes,and we would expect them to produce correspondingly fainteremissions since meniscus motions would involve less energy. Thus,AEs generated during drying in media with rough particles andirregular pores would tend to occur at lower amplitudes and ener-gies than for AEs produced during drying in media whose particlesand pore spaces are smooth.

3.4.2. Power–law relation between AE amplitude and event occurrenceFluid invasion in porous media is frequently modeled as a form

of invasion percolation with pore-scale burst size distributions thatdiminish in occurrence with size. Previous studies have found thatthe statistical distributions of pressure jumps match power–laws(Aker et al., 2000; Måløy et al., 1992; Furuberg et al., 1996) sug-gesting that AE size distributions may behave similarly. The AEamplitude distributions that emerge during drainage from sandand glass beads display similar power law behavior, but drainagehas been shown to yield exponents with lower magnitudes thanthose produced by imbibition (DiCarlo et al., 2003; Moebiuset al., 2012). These exponents describing AE size distributions havealso been shown to be strongly influenced by the particle sizes andthus pore sizes of the host medium (Moebius et al., 2012). Here weshow that similar scaling behaviors also exist in the context ofevaporation and are influenced by particle size, grain shape, andconsequently by pore geometry. Fig. 6 displays the amplitude dis-tributions for each particle type used in the present study. Follow-ing Moebius et al. (2012) a power law of the form N = a10bA[dB] wasfitted to the data where N is the recorded number of AE hits, A isthe waveform amplitude in dB, a is the constant of proportionalityand b describes the exponent of power law.

The curves fitted to these histograms exhibit power lawswherein the number of AEs observed decreases with amplitudefor each particle. In all cases, fainter events are more abundant thanlouder events. Among particles with the same surface roughness,the magnitude of the exponent tends to increase with decreasedparticle size, indicating that small particles preferentially producehigher proportions of faint AEs relative to large particles. Theconverse also holds; higher proportions of loud AEs are generatedduring evaporation from the media with larger particle sizes.

Comparing across grain shapes, we find that the smoothspherical glass beads with an average particle size of 0.53 mmexhibit a power law with a less negative value than the similarlysized but irregularly-shaped sand particles of average particle size0.58 mm – the drying of smoother particles generates AEs withlarger amplitudes. In other words, relative to smooth media, evap-oration from pore spaces with rough, irregular surfaces producesmore numerous AEs albeit at lower amplitudes.

3.4.3. Characteristics of AE hits as influenced by the particle sizeAt a meniscus’ contact line, adhesive forces compete with grav-

itational, capillary, and viscous forces to stabilize the meniscus.

type. The data for (a) S 0.34 mm, S 0.58 mm, and S 0.89 mm are described by beta�0.06 respectively. S and GB refer to sand and glass beads with the average particle

Fig. 7. Hit parameters averaged across all AEs recorded during stage-1 evaporation for each particle type. (a) Blue columns correspond to sand particles and red columns toglass beads. Results show that average hit energies trend upward with particle size and that AEs from glass beads exhibit higher energies than those from similarly sized sandgrains. (b) Hit energy versus amplitude (A) in a double logarithmic scale. Hit energy (E) scale reasonably with the square of hit amplitudes in all cases. The solid linecorresponds to the ideal case where E � A2.

N. Grapsas, N. Shokri / International Journal of Multiphase Flow 62 (2014) 30–36 35

The amount of force needed to overcome adhesion depends on theproperties of the liquid, the wettability of the surface, and poregeometry among other factors. When the meniscus destabilizesand undergoes a Haines jump, it releases energy in the form ofan AE with features influenced by jump dynamics. It is reasonableto assume that hit energies may be imprinted with informationabout the nature of fluid interaction with the surface from whichthe meniscus detaches. Extending this logic, the geometry of thepore space (size and roughness) should affect individual AE wave-forms either by controlling the nature of the de-pinning event orby modifying jump dynamics.

Fig. 7a presents the mean energies observed for samples duringstage-1 evaporation. Typical hit energies increase with particle sizefor both sand and glass beads, a trend aligned with the associatedincreases in pore size and jump displacement. Comparing the AEsfrom glass beads with average particle size of 0.53 mm to thosefrom the sand with average particle size 0.58 mm reveals thatevaporation from beads generates more energetic AEs on averagethan it does from sand. Fig. 7b plots hit energy as a function ofamplitude in units of Pascal for each particle type in log–log space.Since the energy of an acoustic wave scales with the square of itsamplitude (Nakamura et al., 1972), a power law of 2 would emergein the presence of idealized sound propagation. As indicated inFig. 7b, the exponents arising from these experiments deviateslightly from the ideal case, seemingly due to a combination ofAE sensor detection accuracy and the 26 (dB) threshold used to fil-ter out background noise. Since hit amplitude scales with thesquare root of hit energy, it follows that any patterns in hit energywill be reflected in hit amplitude. Accordingly, larger particle sizesand smoother surfaces correlate with increased AE amplitude.

4. Summary and conclusions

Based on the findings of this paper, AE methods appear capableof non-destructively characterizing evaporation from porous med-ia in real time while revealing information about the host mediumand its resident fluid. There might be practical limitations to AEtechniques at this stage, but this study illustrates the potential ofAE methods in the context of drying – they may be used to non-invasively characterize evaporation rates from soil surfaces com-posed of varying particle sizes and grain shapes. This representsa crucial step toward developing a direct method for environmen-tal flux measurements. Future work in this direction could focus onunambiguously delineating the source mechanism of AE genera-tion; establishing a quantitative understanding of precisely howthe acoustic signature generated during drying relates to poregeometry and pore-scale fluid dynamics; and determining whetherthe properties of AE waveforms can be used to deduce total evap-orative mass loss.

Acknowledgements

The authors would like to thank Prof. Dani Or for helpful discus-sions. We would like to thank Prof. Jonathan D. Woodruff and hisgraduate student Christine Brandon at University of Massachu-setts, Amherst for allowing us to use their CAMSIZER Digital ImageProcessing (from HORIBA) to analyze the particle size distributionof the materials used in our experiments.

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