Particle Separation inside a Sessile Droplet with Variable Contact

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Particle Separation inside a Sessile Droplet with Variable Contact Angle using Surface Acoustic Waves Ghulam Destgeer, Jin Ho Jung, Jinsoo Park, Husnain Ahmed, and Hyung Jin Sung Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03314 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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Analytical Chemistry

Particle Separation inside a Sessile Droplet with Variable Contact Angle using Surface Acoustic Waves Ghulam Destgeer, Jin Ho Jung, Jinsoo Park, Husnain Ahmed, and Hyung Jin Sung* Department of Mechanical Engineering, KAIST, Daejeon 34141, Korea. E-mail: [email protected] ABSTRACT: A sessile droplet of water carrying polystyrene microparticles of different diameters was uniformly exposed to high frequency surface acoustic waves (SAWs) produced by an interdigitated transducer (IDT). We investigated the concentration behavior of the microparticles as the SAWs generated a strong acoustic streaming flow (ASF) inside the water droplet and exerted a direct acoustic radiation force (ARF) on the particles, the magnitude of which depended upon the particle diameter. As a result of the ARF, the microparticles were concentrated according to their diameters at different positions inside the sessile droplet placed in the path of the SAW, right in front of the IDT. The microparticle concentration behavior changed as the sessile droplet contact angle with the substrate was varied by adding surfactant to the water or by gradually evaporating the water. The positions at which the smaller and larger microparticles were concentrated remained distinguishable, even at very different experimental conditions. The long-term exposure of the droplets to the SAWs was accompanied by the gradual evaporation of the carrier fluid, which dynamically changed the droplet contact angle as well as the concentration of particles. Complete evaporation of the fluid left behind several concentrated yet separated clusters of particles on the substrate surface. The effect of the droplet contact angle on particles’ concentration behavior and consequent separation of particles has been uniquely studied in this SAW-based report.

Introduction Acoustofluidic actuation of a microfluids1,2 is important for realizing rapid polymerase chain reactions,3,4 and efficient fluid nebulization of a sessile droplet5,6 in the context of drug delivery7–9 and thin film deposition.10,11 The manipulation of suspended micro-objects suspended inside a sessile droplet, such as cells12 or micro- and nanoparticles13,14 is useful in disease diagnosis,15 separation,16 concentration,17–20 and signal enhancement21 applications. With the potential to realize a plethora of biophysical applications,22,23 sessile droplet-based acoustofluidic platforms are simple alternatives to similar microchannel-based platforms24–28 for studies of acoustofluidic physical phenomenon. Interdigitated transducer (IDT)-based technologies can produce relatively high-frequency (10-1000 MHz) surface acoustic waves (SAWs)29–33 compared with the bulk acoustic waves (BAWs)-based technologies34–37 that operate at a lower frequency (1-10 MHz) and relay on the formation of standing acoustic waves. SAWs have the potential to form travelling or standing acoustic fields on-demand or a combination of both,38,39 generate a strong acoustic streaming flow (ASF),40,41 and produce significant acoustic radiation force (ARF) to manipulate suspended micro-objects.41–44 Alternatively, vibrating sharp-edges45,46 and oscillating trapped-bubbles47,48 inside a microchannel are also used to realize efficient microfluidic actuation capabilities inside a microchannel. However, these devices require high input power for their operation and cannot generate desired ARF due to low actuation frequencies. We recently used low-frequency (around 10 MHz) as well as high-frequency (around 100 MHz) SAWs to investigate the concentrating effects of these SAWs on polystyrene (PS) microparticles in a sessile droplet of water eccentrically positioned

on a substrate surface with reference to the IDT.17 We observed four distinct concentrating effects that depended on the particle diameter, SAW frequency, acoustic wave attenuation, complex acoustic field, strong ASF, and direct ARF produced by the travelling or standing waves. In a separate investigation, we built upon the findings of Rezk et al.49 by using Lamb waves (a type of BAWs) to study the concentration of PS microparticles in the form of a ring while monitoring the effects of the direct ARF and the ASF on the motion of the microparticles.50 We summarized the complex phenomenon associated with acoustofluidic handling of microparticles suspended in a sessile droplet using a simple and easy to understand model.17,50 However, the richness and complexity of the phenomenon and several unanswered questions, which were not discussed in the previous reports, compelled us to further study the particle concentration over long SAW exposure time periods and with changing droplet contact angles for the goal of realizing particle separation. In this work, we have studied the variable concentration behaviors of the particles for different contact angles of the water droplet, and demonstrated separation of particles based on their diameter differences. We used a straight IDT to actuate a sessile droplet of water carrying suspended particles under uniform exposure to the SAWs contrary to the eccentric exposure used previouly.50 The high-frequency SAWs (130 MHz), with a shorter attenuation length, produced a strong ASF as they coupled with the sessile droplet and thus transformed into a longitudinal acoustic wave inside the fluid. Dentry et al. have investigated the effects of SAW-frequency and associated attenuation lengths on the ASF produced inside the actuated fluid.51 The use of the high-frequency SAWs in this study precluded the placement of the droplet on the side of the IDT, as was required previously, especially using low-frequency (10 MHz) SAWs with a much longer attenuation length.52–54 The suspended particles

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were concentrated along different shapes and at different locations that depended upon the diameters of the particles, SAW exposure time, and the contact angle of the sessile droplet with the substrate. The droplet contact angle was decreased significantly by the addition of surfactant to water. The contact angle could also be changed dynamically as the liquid gradually evaporated over the long exposure to the SAW-an aspect of the device that was not studied before. The motion of the particles was affected by the ASF and the direct ARF mainly via the travelling longitudinal acoustic waves propagating through the droplet. The propagation and attenuation of the travelling acoustic wave inside the fluid and the subsequent formation of an acoustic field depended significantly on the shape and contact angle of the droplet. The particle concentration behavior changed as the water evaporated upon application of the high frequency SAW (130 MHz). Previously, the concentration and separation of variable size particles have been demonstrated by utilizing the coffee ring effect within a naturally evaporating droplet at room temperature.55 The coffee ring effect has been reversed using Marangoni flow to concentrate the particles at the center of an octane droplet; however, it was observed that the Marangoni flow inside a water droplet was very weak despite the theoretical prediction of a higher flow velocity.56 Recently, the

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concentration of suspended microparticles was shown to be efficiently controlled by the application of low-frequency O(10 MHz) standing SAWs, which suppressed the coffee ring effect as the liquid evaporated, leaving behind solid particle clusters.57 However, the separation of the variable diameter particles by using an external force field (SAWs) has not been reported within an evaporating sessile droplet. A careful analysis of the particle concentration behavior inside an evaporating sessile droplet using high frequency travelling SAWs (130 and 192 MHz) revealed a method of separating microparticles of different diameters influenced by the competing ARF and ASFinduced drag force. A similar mechanism was previously used by Rogers et al.13 to separate 6 and 31 µm particles using 20 MHz SAW; however, the effect of an evaporating droplet with variable contact angle was not studied. The ASF velocity field inside a water droplet was much stronger than the Marangoni flow that allows us to ignore the effect of Marangoni flow used previously to concentrate particles at the center of an octane droplet.56 We further examined the particle concentration behavior as the liquid completely evaporated, resulting in separated clusters of particles of different diameters.

Figure 1. (A) Device schematic: an interdigitated transducer (IDT) was prepared by depositing a pair of gold electrodes onto a piezoelectric lithium niobate (LN) substrate. A sessile droplet of water carrying polystyrene (PS) particles of different diameters was placed in front of the IDT. The surface acoustic wave (SAW) emanating from the IDT interacted with the sessile droplet and produced an acoustic streaming flow (ASF) as a longitudinal wave radiated at the Rayleigh angle (𝜃𝜃𝑟𝑟 ). (B) The sessile droplet of water formed a smaller or larger contact angle with the LN surface, depending on the concentration of added surfactant, which changed the diameter of the droplet. Smaller particles (red), with 𝜅𝜅 < 1, were concentrated in different shapes at different locations compared with larger (green) particles, with 𝜅𝜅 > 1. The separation of red and green particles was possible in both droplets prepared with or without the addition of the surfactant (C).

Experimental

The acoustofluidic platform was constructed simply using a piezoelectric substrate (Lithium Niobate (LN), 128° Y X cut LiNbO3, MTI Korea, Korea) and a pair of interdigitated metal electrodes (IDT, Ti/Au, 50 Å/800 Å) patterned on top using an e-beam evaporation process (IAMD, SNU, Korea) (see Fig. 1(A)).4,17,58 The acoustofluidic device was mounted on the microscope stage (BX53, Olympus, Korea) to observe microparticle motions inside the sample sessile droplet positioned on top of the substrate. The piezoelectric substrate was excited by a resonance frequency AC signal produced by an RF signal generator (N5181A, Keysight Technologies, USA) and amplified by a power amplifier (LZY-22+, Mini Circuits, USA). The experimental images were recorded using an image acquisition

software (CellSens Standard 1.6, Olympus, Korea) and a CCD camera (DP72, Olympus, Korea) attached to the microscope. The built-in microscope light source directed light upward from the bottom of the droplet; the water droplet content was not clearly visible due to the curved hemispherical shape of the sessile droplet and the consequent light refraction. Therefore, an external light source (Mckinley Q5, LEDflash light) was positioned, at an angle with the microscope stage, on the top side of the device to direct light onto the sample water droplet carrying the colored particles. The light reflection at the droplet surface produced two bright spots that were visible in the experimental images on opposite sides of the droplet from where the light entered and left the droplet. Fluorescence images were obtained using the built-in microscope light source and light filters that


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directed and collected single wavelength light beams. The acquired experimental images were analyzed using the ImageJ software (http://imagej.nih.gov/ij/) to create a montage of images captured at different time steps. Fluorescence images of different colors, obtained separately, were also stacked using the ImageJ software. Figure 2 and figures 4-6 were obtained by using the microscope stage and lighting setup explained above. However, to simultaneously capture the top and side views of the sessile droplet (as shown in Fig. 3), we custom-built a stage with two high speed 10 bit CMOS cameras (pco.1200 hs, PCO AG, Germany) aligned perpendicular to each other and equipped with 4x magnification lenses. The device was illuminated by a light source (FOK-100W, Fiber Optic Korea, Korea) through a two port light guide (Dual Type Light Guide, Fiber Optic Korea, Korea) (see Fig. S1).59,60 We used a home-built software to measure the contact angle of the droplet using a three-point measurement model. The sample was prepared by mixing particles with the desired diameters in deionized water (Sinhan Science Tech, Korea) before dispensing a sessile droplet of a specific volume on top of the substrate using a pipet (Eppendorf, USA). The water sample contained 0.0027% solid dyed particles (Polybead® polystyrene microspheres, Polysciences, Inc., USA) with nominal diameters of 3 µm (blue, mean diameter (MD) 3.15 µm) and 6 µm (yellow, MD 5.8 µm). Fluorescence imaging was achieved using fluorescent particles (Fluoro-Max™ polymer microspheres, Thermo Scientific, USA) with nominal diameters of 0.7 µm (red, MD 0.71 µm), 1 µm (blue, MD 1.0 µm), 2 µm (blue, MD 2.1 µm), 3 µm (red, MD 3.2 µm), and 5 µm (green, MD 4.8 µm). Particle samples, obtained from the supplier, contain minimum volume percentage of surfactant required to avoid unnecessary particles’ coagulation. However, after preparing the diluted particle samples in water, additional surfactant was added to lower the droplet contact angle without modifying the substrate surface.

Working mechanism Figure 1 shows a schematic diagram of the acoustofluidic device supporting a sessile droplet of water (A) and the mechanism underlying particle concentration (B) that led to the separation of particles (C) based on size differences. A sample sessile droplet contains PS particles of larger and smaller diameters as indicated by the green and red colors, respectively (see Fig. 1(A)). The SAW (a Rayleigh wave emanating from the IDT and propagating over the surface of the LN substrate) interacted with the droplet in its path of propagation as a longitudinal acoustic wave is radiated at a Rayleigh angle (𝜃𝜃𝑟𝑟 ) inside the droplet. The SAW attenuated across the substrate surface upon interaction with the fluid, and the longitudinal acoustic wave attenuated within the fluid that produced a strong ASF. The ASF moved the particles in two adjacent vortices as an additional ARF acted on the particles in a size-dependent manner. The movements of particles within the water droplet prepared with or without a surfactant (resulting in a low or high contact angle) were clearly distinct, and their concentration positions were readily distinguished based on the particle size (see Fig. 1(B)). A water droplet prepared without surfactant and having a high contact angle produced particle behaviors in which the smaller red particles (characterized by 𝜅𝜅 < 1 , where 𝜅𝜅 = 𝜋𝜋𝑓𝑓𝑆𝑆𝑆𝑆𝑆𝑆 𝑑𝑑𝑝𝑝 /𝑐𝑐𝑓𝑓 is a dimensionless Helmholtz number, 𝑓𝑓𝑆𝑆𝑆𝑆𝑆𝑆 is the SAW frequency, 𝑑𝑑𝑝𝑝 is the particle diameter, and 𝑐𝑐𝑓𝑓 is the speed

of sound in the fluid) formed two rings that approached the periphery of the droplet. The larger green particles (with 𝜅𝜅 > 1) formed a smaller ring or moved in worm-like streams along a three-dimensional pattern within a smaller ring structure as the ARF pushed the particles inward. The relationship between the ASF-induced drag force and the value of 𝜅𝜅 was recently elucidated, as the ARF was found to dominate over the ASF-induced drag force for 𝜅𝜅 > 1 and vice versa.50 A higher contact angle (𝜃𝜃ℎ ) of the droplet produced a threedimensional ASF with significant vertical motions of the particles. A droplet prepared with added surfactant formed a lower contact angle (𝜃𝜃𝑙𝑙 ) that suppressed the vertical motions of the particles, and the ASF formed a two-dimensional flow with additional flow structures close to the frontal region of the droplet. The ASF in both cases (i.e. a sessile droplet with or without surfactant) can be attributed to the Eckart streaming61 as the acoustic wavelengths in the LN substrate (𝜆𝜆𝑆𝑆𝑆𝑆𝑆𝑆 ~ 30 𝜇𝜇m) and inside the fluid (𝜆𝜆𝑆𝑆𝑆𝑆𝑆𝑆 ~ 12 𝜇𝜇m) are much smaller than the sessile droplet diameter (~ 3 mm) and height (~ 0.75 mm). The height of the droplet gradually decreases upon evaporation; however, it only becomes comparable with the acoustic wavelength at the very later stages of the evaporation when the contact angle recedes below ~ 5°. Therefore, the possibility of boundary layer acoustic streaming taking over the Eckart streaming could be considered for smaller fluid thicknesses, especially close the periphery of the droplet.10,62 The smaller particles formed two large rings in the main droplet body and concentrated in the vortices present in the frontal region of the droplet. The larger particles, however, formed smaller rings and could also be pushed to the rear of the droplet under the influence of the direct ARF. The ASF could drag the larger particles back to the ring structures or push them to the center of the vortices. The larger particles were absent from the frontal region of the droplet because the ARF was strong enough to push them along the direction of SAW propagation. The smaller and larger concentrated particles occupied distinct locations in both cases, i.e. in the droplets prepared without or with the surfactant to form higher or lower contact angles, leading to the size-dependent separation of particles, as shown in Fig. 1(C).

Results and discussion Figure 2 demonstrates the manipulation of suspended microparticles, via SAWs 130 MHz in frequency and 213 mW in input power, inside a sessile droplet 2.5 µL in volume prepared without (A-C) or with surfactant (D-F). The water droplet was 2.25–2.75 mm in diameter on the substrate surface and increased to 3–4 mm upon the addition of 0.1% v/v surfactant. At a constant droplet volume, additional surfactant significantly lowered the contact angle. Assuming the droplet always formed a shape similar to a spherical cap, we estimated that a 2.5 µL droplet formed a contact angle of 70.01° (𝜃𝜃ℎ ) or 32.23° (𝜃𝜃𝑙𝑙 ) for cap base radii of 1.25 mm or 1.75 mm and cap heights of 0.88 mm or 0.51 mm, respectively.63 We experimentally measured the contact angle (using the setup shown in Fig. S1) for a water droplet without additional surfactant placed on a LN substrate to be 70.86 ± 9.22° at a room temperature of 25.43 ± 0.89°C and humidity of 42.56 ± 2.72%, which changed to 57.12 ± 5.77° at a room temperature of 21.01 ± 0.15°C and humidity of 46.88 ± 0.35%. The contact angle of a water droplet with 0.1% v/v surfactant was measured to be 21.10 ± 5.45° at a room temperature of 21.2 ± 0.00°C and humidity of 46.63 ± 0.50%. The ASF



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field changed significantly with the contact angle, as did the concentrating behavior of the particles.

Figure 2. Microparticles with a diameter of 3 µm (blue) or 6 µm (yellow) were suspended in a 2.5 µL water droplet prepared without surfactant (A–C) or with surfactant in a 0.1% volume ratio (D–F). The droplets with suspended particles were exposed to a 130 MHz SAW at a 213 mW input power, propagating from top to bottom as indicated by orange arrow. (A) The smaller 3 µm particles formed two rings after 60 s of SAW exposure. (B) The larger 6 µm particles formed a smaller ring within 10 s, as the ARF was dominant. (C) The separation of 3 and 6 µm particles was evident after 60 s of SAW exposure. (D) A larger ring was formed in the central region with the concentration of particles in the front region after 30 s. (E) The particles formed two relatively smaller rings within 10 s. (F) Separation of the particles was achieved after 57 s as the smaller particles formed a ring and were concentrated in the front region of the droplet, and larger particles were pushed inward to form an even smaller ring, were concentrated at the core of the vortex, or were pushed toward the rear of the droplet. The scale bars indicate 1 mm. See Movie I in the Supporting Information: SAW is propagating from left to right.

The water droplet prepared without additional surfactant and with 3 µm suspended blue particles formed two ring-like structures of concentrated particles upon exposure to a SAW for 60 s (see Fig. 2(A)). The larger 6 µm yellow particles were concentrated in a smaller ring over a much shorter time period of 10 s (see Fig. 2(B)). The positions of the smaller 3 µm and larger 6 µm particles were readily distinguished. A droplet with suspended particles of 3 and 6 µm diameters was exposed to a SAW that separated the particles as the majority of smaller particles were concentrated on the right side in a large ring-like shape and the larger particles were concentrated on the left side of the droplet (see Fig. 2(C)). In a water droplet prepared with 0.1% v/v surfactant, the 3 µm blue particles were concentrated in small packets in the front region of the droplet and formed a ring in the central region after 30 s of SAW exposure (see Fig. 2(D)). The larger 6 µm yellow particles, on the other hand, were absent from the front region of the droplet and formed a relatively smaller rings compared to the 3 µm particles due to the strong ARF that pushed them inward over 10 s of SAW exposure (see Fig. 2(E)). A mixture of 3 µm blue and 6 µm yellow particles could be separated as the droplet was exposed to a SAW over 57 s (see Fig. 2(F)). The 6 µm yellow particles were concentrated within a larger ring of the 3 µm blue particles and were pushed to the rear of the droplet as the contact angle decreased due to liquid evaporation. Figure 3 displays the top (A, C) and side (B, D) views of the sessile droplet with suspended PS particles as the water evaporated and the droplet contact angle gradually decreased, thus changing the particle concentration behavior. The PS particles with 3 µm diameter, suspended inside a 2.5 µL droplet, were exposed to 130 MHz SAWs at 851 mW input power (see Fig. 3(A) and 3(B) for top and side views, respectively). The droplet evaporation rate was measured to be ~ 0.27 µL/min at a room temperature and humidity of 25.9°C and 42%, respectively. The particles formed two adjacent and symmetrical rings after 80 s

of SAW exposure as the contact angle was 59°. At 160 s, the contact angle decreased to 50°; however, the two rings retained their shape. As the contact angle further decreased to 40° (at 240 s), the particles ring-structure was distorted because the ASF patterns changed and the particles started concentrating in the front region of the droplet. Decreasing contact angle (29° to 2° from 300 s to 480 s) brought more particles in the front region as the two ring-like shapes disappeared. We exposed another 2.5 µL sessile droplet, with 6 µm diameter suspended PS particles to 130 MHz SAWs at 1.33 W input power (see Fig. 3(C) and 3(D) for top and side views, respectively). The droplet contact angle decreased from 56° to 6° (from 4 s to 404 s) as the liquid evaporated rate was ~ 0.33 µL/min at a room temperature and humidity of 21.1°C and 47%, respectively. As the ARF dominated the particle motion, the particles moving in a threedimensional pattern (at 4 s and 56° contact angle) were concentrated at the core of ASF vortices (at 84 s and 164 s, and 43° and 35° contact angle). Further decrease in the contact angle to 26° and below resulted in most of the particles being pushed in the rear part of the droplet at 244 s onward (see Fig. S2 in the Supporting Information for the additional results). It has been observed that the particle concentration behavior significantly changed as the droplet contact angle receded below ~ 30 ± 5° and approached the Rayleigh angle (~ 22° obtained using Snell’s Law for a SAW coupling into a water droplet from LN substrate). Figure 4 presents the change in the particle-concentrating behavior as the water gradually evaporated and lowered the droplet contact angle. Evaporation rates of 0.5-0.9 µL/min for 2.510 µL droplets were observed for 479 mW input power and 130 MHz frequency. The rate increased with the droplet volume as the surface area of the evaporating liquid increased. The smaller 3 µm blue particles were concentrated in a single (at 60 s) or double (at 180 s) ring-like structures that transformed into a larger ring (at 300 s) as the contact angle decreased because of


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liquid evaporation at ~ 0.7 µL/min (see Fig. 4(A) and Fig. S3 in the Supporting Information for the additional results). The change in contact angle could be monitored by the outward movement of a bright spot on the droplet created by the light reflection, as indicated by the arrow. The larger 6 µm yellow particles were concentrated close to the center of the two ASF vortices (at 100 s), forming a planar ring-like structure (at 200 s) as some of the particles were pushed to the rear of the droplet

(at 300 s) for evaporation rate of ~ 0.9 µL/min (see Fig. 4(B)). Additional results are obtained using lower (13 mW) as well as higher (850 mW) input powers to concentrate 6 μm yellow particles (see Fig. S4 in Supporting Information). As the contact angle further decreased upon continued water evaporation, more and more particles were pushed to the rear of the droplet (from 400 s to 500 s), while the remaining particles either formed a ring or were concentrated at the core of the vortex.

Figure 3. Top views (A, C) and side views (B, D) of the sessile droplets are shown to demonstrate temporal progression of particle concentration behavior as the liquid continuously evaporated, lowering the contact angle and changing the internal streaming flow structure. (A) PS particles with 3 µm diameter, suspended in 2.5 µL droplet, are exposed to 130 MHz SAWs at 851 mW input power (top view). (B) Side view, corresponding to (A), shows that the contact angle decreased from 59° to 2° as the exposure time increased from 80 s to 480 s. Room temperature and humidity were measured to be 25.9°C and 42%, respectively. (C) PS particles with 6 µm diameter, suspended in 2.5 µL droplet, are exposed to 130 MHz SAWs at 1.33 W input power (top view). (D) Side view, corresponding to (C), shows that the contact angle decreased from 56° to 6° as the exposure time increased from 4 s to 404 s. Room temperature and humidity were measured to be 21.1°C and 47%, respectively. The images are captured at magnification 4x, while the scale bars indicate 500 µm. See Movie II in the Supporting Information.

Figure 4. The temporal progression of the particle concentration behaviors is shown as the water gradually evaporated and, consequently, the contact angle decreased. (A) The concentration behavior shifted from a two-ring structure to a more flattened single larger ring upon exposure of a 7.5 µL droplet of water with 3 µm blue particles was exposed to 130 MHz SAWs for 300 s. (B) The lager 6 µm yellow particles, suspended in a 10 µL water droplet, initially formed a smaller ring (at 100 s and 200 s). As the contact angle of the droplet decreased due to evaporation, the particles were either concentrated at the center of the ring or gradually pushed to the rear part of the droplet by the strong ARF. The scale bars indicate 1 mm. See Movie III in the Supporting Information.



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Figure 5 illustrates the separation of particles as the liquid continued to evaporate, eventually leaving behind concentrated islands of particles of different diameters. The particles suspended inside the water droplet formed distinct blue and yellow rings based on their diameters (at 180 s) upon exposure to a low input power of 53 mW, while the evaporation rate was ~ 0.2 µL/min (see Fig. 5(A)). The larger particles continued to be engulfed by the smaller particle rings at 360 s and 540 s. As the contact angle decreased further, the smaller particles became concentrated at the front region of the droplet, whereas the larger yellow particles were concentrated at the core of the ASF vortices at 720 s and 900 s. A higher input power of 479 mW produced a clear separation of particles at 90 s as the evaporation rate increased significantly to ~ 0.6 µL/min (see Fig. 5(B)). The particles passed through a transitory contact angle change phase at 270 s, and most of the larger particles were pushed to the rear of the droplet by 360 s whereas the smaller particles remained present in the frontal region of the droplet. At 450 s, the evaporating liquid boundary gradually dragged the smaller particles to the rear of the droplet, where the larger particles had anchored the droplet boundary against recession. Once the liquid had fully evaporated, the particles formed scattered islands on the surface (see Fig. 5(C) and 5(D), which correspond to 5(A) and 5(B)). At a low input power, some the larger particles

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concentrated at the core of the ASF vortices retained their positions as the remaining larger particles were pushed forward toward the peripheral region (see Fig. 5(C)). Closer inspection at 4x or 10x magnification revealed several small blue particles present among the larger yellow particles. This feature was attributed to the vibrations of the larger yellow particles, which generated secondary radiation forces that attracted nearby smaller particles toward them.64 The receding boundary of an evaporating droplet carried the scattered smaller blue particles that were unaffected by the ARF toward the concentrated yellow particles that had settled on the substrate surface and anchored the droplet. At a higher input power, the central particle islands were absent because the strong ARF had already pushed most of the larger yellow particles toward the back peripheral region, and the smaller blue particles followed the evaporating droplet boundary and were stacked along with the larger particles in a separate layer clearly visible at higher magnifications of 4x or 10x (see Fig. 5(D)). Separation of 3 μm blue particles and 6 μm yellow particles is further demonstrated at different input powers to confirm the repeatability of the phenomenon (see Fig. S5-S7 in Supporting Information). Moreover, temporal progress of the separation of particles is also demonstrated inside a sessile droplet with 0.1% v/v surfactant (see Fig. S8 in supporting information).

Figure 5. The temporal progression of particle separation is presented as the liquid continuously evaporated, lowering the contact angle and changing the internal streaming flow structure. (A) Separation of 3 µm blue and 6 µm yellow particles was realized at a significantly lower input power of 53 mW over a time period of 900 s. (B) The separation of particles and complete evaporation of water was achieved at 450 s at an input power of 479 mW. All larger particles were pushed to the back of the droplet as the higher input power ensured a strong ARF throughout the droplet. (C, D) The concentrated particles after water evaporation, corresponding to (A) and (B), are shown at different magnifications 2x, 4x, and 10x as the scale bars indicate 1 mm, 500 µm, and 200 µm, respectively. See Movie IV in the Supporting Information.

Figure 6 displays the separation of several different pairs of fluorescent particles exposed to 130 MHz or 192 MHz SAWs. A sessile droplet of water carrying fluorescent blue 1 µm and green 5 µm particles was exposed to a 130 MHz SAW at a 2.25 W input power (see Fig. 6(A)) to realize particle separation within 15 s as the blue particles formed two rings that were distinct from the concentrated green particles on one side of the droplet, within one of the blue rings (see Fig. 6(B)). Time periods of 58 s or 40 s were required to realize similar degrees of separation at input powers of 213 mW or 479 mW, respectively (see Fig. 6(C) and 6(D)). Similarly, in a droplet with a lower contact angle, the separation of particles was achieved as the green particles formed a smaller ring-like structure whereas the

blue particles were concentrated in the front region at input powers of 213 mW or 479 mW and exposure times of 55 s or 35 s, respectively (see Fig. 6(E) and 6(F)). The separation of 0.7 µm red and 5 µm green particles was more apparent in Fig. 6(G) and 6(H), as the smaller particles formed a ring larger than the green ring; moreover, the red particles were concentrated in the front region of the droplet. The 130 MHz SAWs at an input power of 851 mW induced the formation of similar rings of the 2 µm blue and 3 µm red particles, despite a clear size difference, because 𝜅𝜅 < 1 for both particles, and the ARF was insignificant (see Fig. 6(I)). The same 130 MHz frequency SAWs imposed separation between 3 µm red (𝜅𝜅 < 1) and 5 µm green (𝜅𝜅 > 1)


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particles as the ARF became significant (see Fig. 6(J)). By contrast, particle manipulation using a 192 MHz SAW with a 1.92 W input power did not separate 3 µm red (𝜅𝜅 > 1) and 5 µm green (𝜅𝜅 > 1) particles because the motions of both particles were dominated by the ARF (see Fig. 6(K)); however, the 2 µm blue (𝜅𝜅 < 1) and 3 µm red (𝜅𝜅 > 1) particles could be separated under these conditions (see Fig. 6(L)). These results highlighted the importance of appropriately tuning the frequency to realize separation of a given pair of particles such that one particle is dominated by the ARF and the other particle motion is driven

by the ASF only. The separation of 2 µm fluorescent blue from 3 µm fluorescent red particles inside a droplet with a low contact angle was also demonstrated using focused SAWs with a frequency of 192 MHz (see Fig. S9 in Supporting Information). We used a focused-SAW (129 MHz) to observe the concentration behavior of 5 µm green fluorescent particles with varying concentrations in sessile droplets of different volumes (2.5, 5, 7.5, 10 µL) and at different input powers (13, 53, 213 mW) and surfactant ratios (see Fig. S10-S13 in Supporting Information).

Figure 6. (A–F) The separation of fluorescent blue 1 µm and green 4.8 µm particles was demonstrated using a 130 MHz SAW with different input powers and variable exposure times. (G–H) The separation of fluorescent red 0.71 µm and green 4.8 µm particles in a droplet with a lower contact angle. (I) Fluorescent blue 2.1 µm and red 3.2 µm particles exposed to a 130 MHz SAW were not influenced by the ARF; therefore, both particles formed similar ring-like structures and were not separated. (J) The separation between 3.2 µm red and 4.8 µm green particles was induced by a 130 MHz SAW, as the larger particles experienced a strong ARF that differentiated them from the smaller red particle rings. (K) For a mixture of 3.2 µm red and 4.8 µm green particles exposed to a 192 MHz SAW, the separation was not clear, as both of the particles were dominated by the ARF and did not form distinct concentrated rings. (L) However, the 192 MHz SAW induced separation of 2.1 µm blue particles from 3.2 µm red particles, as the smaller particles were not influenced by the ARF. of a single droplet of biological fluid e.g. blood for point-of-care disease diagnostics.

Conclusions We experimentally investigated the microparticle concentration behavior inside a sessile droplet using SAWs with frequencies of 130 or 192 MHz. We demonstrated PS particles’ separation based on the differences in their diameters upon excitation of a sessile droplet by a high-frequency travelling SAW to induce a strong ASF and impart a direct ARF on the particles. The first observation of the simultaneous particle concentration and separation by a travelling acoustic wave was made as the water droplet gradually evaporated, thereby changing the contact angle of the droplet. Dynamic contact angle changes significantly affected the particles’ motion. A transition in particle concentration behavior was observed when the droplet contact angle receded below ~ 30 ± 5° as the smaller particles’ rings changed their shapes and the larger particles were pushed to the rear of the droplet. We observed a droplet excited by a SAW over an extended period, up to 15 min, to characterize the particles’ concentration behavior. Changes in the contact angle of the sessile droplet resulted in different forms of the ASF patterns and, consequently, variations in the particle concentration and separation behavior. The droplet contact angle could also be readily lowered by adding surfactant to the droplet, which produced a similar particle concentration behavior observed inside a droplet with dynamic contact angle variation due to evaporation. We separated 3 µm blue and 6 µm yellow particles using 130 MHz SAWs inside a droplet with or without additional surfactant. As the water fully evaporated, it left behind concentrated islands of different diameter particles that could be easily distinguished. An acoustofluidic platform of this type is expected to come in handy for the manipulation

ASSOCIATED CONTENT Supporting Information Additional schematic and results are provided in the form of a PDF file and four movie files. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Author Contributions G.D. conceived the research. H.J.S. supervised the research. G.D., J.H.J., and J.P. designed the experiments. G.D., and H.A. performed the experiments. G.D., J.H.J., J.P., H.A., and H.J.S. contributed to the manuscript preparation. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the Creative Research Initiatives (no. 2016-004749) program of the National Research Foundation of Korea (MSIP), the KUSTAR-KAIST Institute, and KAIST-funded K-Valley RED&B Project (2016).



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Table of Content Graphic


Particle Separation inside a Sessile Droplet with Variable Contact

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