Remote sensing of pressure inside deformablemicrochannels using light scatteringin Scotch tapeKYUNGDUK KIM,1 HYEONSEUNG YU,1 JOONYOUNG KOH,2 JUNG H. SHIN,2

WONHEE LEE,2 AND YONGKEUN PARK1,*1Department of Physics, KAIST, 291 Daehakro, Daejeon 305-701, South Korea2Graduate School of Nanoscience and Technology, KAIST, 291 Daehakro, Daejeon 305-701, South Korea*Corresponding author: yk.park@kaist.ac.kr

Received 7 December 2015; revised 12 March 2016; accepted 12 March 2016; posted 14 March 2016 (Doc. ID 255243); published 14 April 2016

We present a simple but effective method to measure thepressure inside a deformable microchannel using laser scat-tering in a translucent Scotch tape. Our idea exploits thefact that the speckle pattern generated by a turbid layer issensitive to the changes in the optical wavefront of an im-pinging beam. A change in the internal pressure of a channeldeforms the elastic channel, which can be detected by meas-uring the speckle patterns of a coherent laser beam that haspassed through the channel and the Scotch tape. We dem-onstrate that with a proper calibration, internal pressurecan be remotely sensed with the resolution of 0.1 kPa withina pressure range of 0–3 kPa after calibration. © 2016OpticalSociety of America

OCIS codes: (280.0280) Remote sensing and sensors; (280.5475)

Pressure measurement; (290.5820) Scattering measurements;

(290.5880) Scattering, rough surfaces.

http://dx.doi.org/10.1364/OL.41.001837

Gas flows in microchannels are one of the important compo-nents for the efficient transport of matter or energy in micro-electro-mechanical systems (MEMS) [1]. Over the past decades,a variety of gaseous systems has been miniaturized into micro-channel systems, such as a cooling system [2] and a gas analyzer[3]. Gas dynamics in microchannels is also a fundamentally in-teresting physical phenomenon [4]. Various experimental andtheoretical studies have been performed [5,6] to effectively utilizethe gas inside such micro-fabricated channels.

In the study of gas [7] flows in microchannels, pressure playsan important role in characterizing or controlling the gaseousenvironment inside. To engineer the gas flow inside the chan-nel, monitoring pressure is essential [8]. One of the approachesfor direct pressure sensing is to create a hole in the channel andinsert a needle-like tip of pressure gauge, but doing so candramatically affect the pressure. To avoid direct sensing, theexternal transducers [9] and microfabricated built-in sensors[10] were proposed. However, these methods are not cost-effective, especially given their low resolutions [11].

Alternatively, various optical methods to measure pressurehave been developed. One type of method is based on opticalfiber sensors, such as Mach–Zehnder interferometry [12],Fabry–Perot etalon [13], and fiber Bragg grating [14,15].They can provide high sensitivity of detecting the pressure-in-duced deformation in order of a wavelength. However, the useof fiber sensors with microchannels is usually restricted due tothe difficulty of integrating them with the channel of a smallsize. In order to gauge pressure inside deformable channels non-invasively, a diffraction grating embedded in channels [16] andthe interferometric measurements of channel membrane [17]were also proposed. Unfortunately, it is necessary to apply com-plicated external measurement schemes for these methods.

Herein, we propose a simple optical method to measure thegas pressure inside a deformable microchannel. This methodexploits speckle patterns formed when a coherent laser beamthat has passed through the transparent channel, whose internalpressure is to be measured, is scattered by a scattering layer. Aminute distortion in the optical wavefront of a laser beam, as-sociated with the deformation of a channel due to the internalpressure, can result in a large change in the speckle patterns.High sensitivity is gained due to the randomness of the geomet-ric structure of the scattering layer, enabling the change in inter-nal pressure to be easily detected by an ordinary image sensor.

Our idea exploits the scattering of light from a disorderedmedium. Random scattering occurs when light encounters amedium with inhomogeneous distribution of refractive in-dexes, or a turbid medium. Due to the complexity of scattering,tiny differences in the incident light are conveyed and amplifiedin the transmitted field [18].

Historically, speckle patterns have been utilized for preciseoptical sensing of physical properties of scattering media such asdisplacement [19], roughness [20], velocity [21], temperature[22], solution concentration [23], and bacteria suspension [24].Conversely, speckle patterns can also be used to analyze theproperties of incident light such as incident angle changes [25],wavelength spectrum [26–28], and images [29–38] instead. Tothe best of our knowledge, however, there has been no investigationfor sensing pressure inside a microchannel using laser speckle [7].

Letter Vol. 41, No. 8 / April 15 2016 / Optics Letters 1837

0146-9592/16/081837-04 Journal © 2016 Optical Society of America

The principle of the present method is illustrated in Fig. 1.To measure the pressure inside a deformable channel, a colli-mated laser beam makes a pass through a transparent micro-channel, below which a layer of Scotch tape is inserted as thescattering medium. The optical phase delay of the beam afterbeing transmitted through the channel is determined by thegeometric shape and refractive index of the channel [Fig. 1(a)].The interior wall of a deformable channel expands when theinternal pressure inside the channel increases. Thus, the phaseof the transmitted beam now contains the information relevantto the internal pressure inside the channel [Fig. 1(b)]. In gen-eral, however, the change in the phase delay is small so that it isbarely observable by a conventional imaging system. However,the scattering layer, by further scrambling and diffusing thelight to form random speckle patterns via interference, greatlyamplifies this difference such that the change in the specklefield can easily be detected by a simple image sensor.

In order to demonstrate proof of these concepts, we imple-mented a setup as illustrated in Fig. 1(c). It consisted of theparts for controlling and measuring the pressure inside a chan-nel, and the optical part to generate and detect speckle patternsfrom a scattering layer. We controlled the pressure using asyringe (Kovax-Syringe 30 ml, Korea Vaccine Co., Republicof Korea), whose volume was regulated with a syringe pump(PHD ULTRA CP 4400, Harvard Apparatus, USA) by the in-fusion and withdrawal of the syringe. To monitor the internalpressure directly as a reference, we directly connected a pressuresensor (PG-30, Copal Electronics, USA) to the syringe bodywith a tube with an inner diameter of 1/16 in. (Teflon PFAtubing, Dupont, USA). The resolution of the reference pressuresensor is 0.01 kPa, and the maximum limit of measurable pres-sure difference is 3.5 kPa. We used microchannels made withpolydimethylsiloxane (PDMS), which is a transparent and defor-mable polymer. The channel dimensions were 30 mm �length�×4 mm �width� × 100 μm �height�, as shown in the inset ofFig. 1(c). The two ends of the microchannel were connected

with tubes with an inner diameter of 1/16 in. The two ends ofthe tubes were then connected by a T-shaped connector, toavoid the pressure gradient inside the channel. At the otherend of the connector, the syringe was connected so that thepressure inside the microchannel could be controlled with thesyringe pump.

For a coherent illumination source, we used a diode-pumpedsolid state laser (λ � 532 nm, 50 mW, Shanghai Dream LaserCo., Shanghai, China). The laser beam was incident on the sur-face of a PDMS microchannel. A scattering layer, made by at-taching a single layer of a Scotch tape (Scotch Magic Tape, 3M,USA) on a slide glass, was located under the channel with a sep-aration of 2 cm. Then the light that transmits through the chan-nel will diffuse out by the scattering layer. The scattering layercan also be attached to the channel. The resulting speckle pat-terns were recorded by a CCD image sensor (INFINITYlite,Lumenera, USA). The distance between the scattering layer andthe CCD was adjusted so that one speckle spot contains approx-imately 10 × 10 pixels in order to maximize the SNR throughtrial and error approach. The SNR can be potentially improvedby using a low noise CCD and optimizing the speckle grainsize [39].

To demonstrate the capability of the present method, wesystemically measured speckle patterns at various internal pres-sures that were controlled by the syringe pump and monitoredby a reference pressure sensor [Fig. 2]. The pressure was ini-tially set to atmospheric pressure, and monotonically increasedand then decreased at a constant rate of 0.09 kPa/s until it re-turned to the initial pressure. At the same time, the speckleintensity patterns of the transmitted beams were recorded bythe CCD sensor. Figures 2(a)–2(h) present the representativebeam patterns at various internal pressures in the absence andthe presence of the Scotch tape.

In the absence of the Scotch tape, the images show beamprofiles with weak diffraction patterns caused by the channel

Fig. 1. (a) Schematic diagram that describes the change in wavefrontof the laser beam, which passed through a transparent channel and ascattering layer. (b) Deformation of wavefront caused by increase ofinternal pressure, and role of scattering layer that amplifies the wave-front difference. (c) Experimental setup.

Fig. 2. Recorded beam intensity patterns at several values of pres-sure in the microchannel, (a)–(d) with and (e)–(h) without a Scotchtape. Speckle pattern is observed when laser beam is scattered by scotchtape. (i) Change of correlation coefficient when the pressure was in-creased and then decreased.

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structure. These images do not vary significantly as pressure isvaried [Figs. 2(a)–2(d)]. When a layer of Scotch tape is placedbeneath the microchannel, speckle patterns appeared. More im-portantly, they change significantly to even a small change inpressure [Figs. 2(e)–2(h)]. To quantitatively analyze the changein speckle patterns, we calculated the correlation coefficient Cbetween two pixelated two-dimensional images of equal size, I 1and I 2, defined as follows:

C �P

m;n�I 1�m; n� − I 1��I 2�m; n� − I2�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�Pm;n

�I1�m; n� − I 1�2��Pm;n

�I 2�m; n� − I 2�2�r ; (1)

where I�m; n� is the intensity at themth row and nth column ofI , I is its mean value, and the range of summations is the entireimage. Figure 2(i) shows the correlation coefficients as a func-tion of time. The correlation coefficient at a given pressure wascalculated with the initial image at 0 kPa. The reference pres-sure (the black line) exhibits a small hysteresis near the peak dueto the backlash of a syringe pump.

Correlation coefficients with and without the Scotch tapeboth follow the trends of a pressure change: they decrease whenthe pressure increases, and rise again as the pressure decreases.Notably, with the Scotch tape, the correlation coefficientchanges more sensitively with the pressure change than it doeswithout the Scotch tape. At 3.5 kPa, the maximum pressure inthis measurement, the correlation coefficient with the Scotchtape drops to 0.34, while it was 0.97 without the Scotch tape.This result clearly shows that a layer of Scotch tape causeslight diffusion that results in an effective conversion of wave-front change to the intensity change via light interference.Furthermore, the symmetry of the pressure-correlation curvesin Fig. 2(i) implies that a similar speckle pattern would beobserved at an arbitrary pressure irrespective of the history.

In order to demonstrate the capability of the present methodas a practical sensor, we addressed the repeatability and hyste-resis of the pressure sensing scheme. In Fig. 3(a), five repeatedmeasurements were performed in which internal pressure wasincreased from 0 to 3 kPa. Measurements of C are highlyrepeatable, with a repeatability defined to be the ratio of thelargest error of C to the averaged C at the given pressure, of1.5% for pressures up to 1 kPa. Above 1 kPa, C increases due tothe decrease in the value of C .

To study the hysteresis of the present method, we comparedthe P-C curves between increasing and decreasing pressure.Figure 3(b) shows that the present method exhibits slighthysteresis; P-C curves for increasing P (the red lines) show

C values higher than those for decreasing P (the black lines).We attribute this hysteresis in C measurements to the hysteresisof the channel deformation.

To further investigate the performance of the presentmethod, we quantified the resolution and hysteresis at variousmeasurements of time intervals between consecutive acquire-ments of speckle images. To quantitatively compare the hyste-resis, we first determined the pressure at which when C becomes0.5 as the pressured is increased/decreased [Fig. 4(a)]. We de-fined the hysteresis to be the difference in pressure for theincreasing and decreasing states [inset, Fig. 4(a)]. In addition,we defined the resolution as the maximum error range in pres-sure at an arbitrary pressure, reflecting both hysteresis and repeat-ability. Figure 4(b) shows the hysteresis calculated at various timeintervals. As the measurement of time interval increases, the hys-teresis tends to become smaller. This is due to the viscoelasticproperty of PDMS, which requires some amount of time torestore its original shape after deformation. It also implies thata microfluidic channel with different viscoelasticity requiresrecalibration.

As shown in Fig. 4(c), the averaged resolution ranged be-tween 0.05 kPa and 0.12 kPa. Resolution increases as the mea-surement of time interval increases, mainly due to the decreaseof hysteresis. Compared to the reported resolution of existingoptical fiber sensors for pressure such as 0.11 kPa for FiberBragg Grating type [40] and 0.69 kPa for Fabry–Perot type[41], the present method achieved a better resolution with asignificantly simpler and more cost-effective setup.

To further investigate the effect of different types of scatter-ing layers on the performance of the present method, we testedmultilayered Scotch tape and 15° diffuser [Fig. 4(d)]. As ex-pected, the P-C curve with a single layer of Scotch tape showssignificant enhancement in performance over that without theScotch tape. Interestingly, increasing the number of tape layersdoes not significantly increase the sensitivity in pressure mea-surement (p-value > 0.4). There was even no difference when

Fig. 3. (a) Correlation curves of repeated measurements, increasingpressure from 0 to 3 kPa. (b) Hysteresis curve with error bars of re-peated measurements. Inset represents the expansion of a rectangularbox in the original figure.

Fig. 4. (a) The change of pressure at which the correlation coeffi-cient between speckle images is halved due to the change in timeinterval between consecutive measurements. Inset represents the def-inition of hysteresis and resolution. The change of (b) hysteresis and(c) resolution when the time interval between consecutive measure-ments was changed. (d) Comparison between correlation curves withdifferent thickness and type of a diffusive layer (error bars; standarddeviations of n � 5 ).

Letter Vol. 41, No. 8 / April 15 2016 / Optics Letters 1839

the type of scattering layer is replaced from Scotch tape to 15°diffuser (p-value > 0.5). This indicates that the method pre-sented in this Letter does not require a particular scatteringmedium to be valid.

In this Letter, we presented a novel optical technique to mea-sure the pressure inside a deformable microchannel. Exploitinglight scattering resulting from translucent Scotch tape, a smallchange in optical wavefront due to internal pressure can resultin a significant change in speckle patterns, allowing the detectionof the internal pressure using a cost-effective but precise method.This is analogous to the concept of phase contrast microscopy,which converts a small change in the wavefront due to a phaseobject into a significant change in the intensity pattern via inter-ference using a phase mask [42]. In previous experiments dealingwith light transport through complex media, small phase changeswere either avoided or minimized [43,44]; however, this workrather utilizes this phenomenon for effective pressure sensing.

The present method can be readily implemented to existingmicrochannel settings because the addition of a Scotch tapelayer and pressure calibration are all that are required. As onlythe speckle patterns need to be measured, the overall detectionsetup can be made simpler than other techniques such as op-tical fiber sensing techniques that require a bulky and expensivespectrometer without sacrificing sensitivity. For example, theuse of a compact laser diode and an image sensor can be usedto achieve the present method. In addition, this method enablesremote in situ sensing of pressure, which may be critical forapplications in which samples may not contact the sensor dueto their reactivity or toxicity. Furthermore, the present methodallows pressure measurements at various points of channel bysimply illuminating a laser beam to the target point. One limi-tation of this method is that it needs a calibration process foreach experimental setting because the nonlinear P-C curves arehighly dependent on the deformability of a channel. However,this limitation can be overcome using a channel with standard-ized elasticity.

Moreover, another popular application of microchannelsis a microfluidic system. The proposed scheme can also be ex-tended to measure the pressure of liquid inside microfluidics.However, we expect that the proposed sensor can be imple-mented in liquid with proper adjustment of system parameters.

With the demonstrated simple implementation and highsensitivity, we envision the present method having direct anddiverse applications in which measuring pressure is important.

Funding. National Research Foundation of Korea (NRF)(2015R1A3A2066550, 2014K1A3A1A09063027, 2012-M3C1A1-048860, 2014M3C1A3052537, 2014M3A6B3063708).

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