Nanoparticle manipulation within a microscale ... - Springer Link

J Nanopart Res (2012) 14:1223 DOI 10.1007/s11051-012-1223-8

RESEARCH PAPER

Nanoparticle manipulation within a microscale acoustofluidic droplet James David Whitehill • Ian Gralinski Duncan Joiner • Adrian Neild

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Received: 30 July 2012 / Accepted: 26 September 2012 / Published online: 16 October 2012 Ó Springer Science+Business Media Dordrecht 2012

Abstract Manipulation of nanoparticles suspended in a droplet can be achieved through the use of low frequency vibration. Upon actuation two types of flows are present in the droplet, a first order flow at the frequency of vibration, and a second order flow in the form of acoustic streaming which is steady state in nature. The former causes collection of nanoparticles, the latter causes swirling of nanoparticles. Through consideration of the two effects, this paper shows that a reduction in fluid height and an associated slight rise in actuation frequency combine to make it possible to manipulate particles in the order of a few hundred nanometers at an excitation frequency of approximately 200 Hz. The amplitude of excitation also plays a role; the increase of amplitude heightens acoustic streaming so this parameter cannot be used as a shortcut to improved ability to collect nanoparticles, hence the need for the analysis of the role of droplet height is presented here. Keywords Vibration Nanoparticles Manipulation Trapping

J. D. Whitehill I. Gralinski D. Joiner A. Neild (&) Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC 3800, Australia e-mail: [email protected]

Introduction Nanoparticle handling and manipulation has been a key focus for the nanotechnology field. This encompasses a range of approaches which aim at a reduction in scale of biological and chemical processes. Nanoparticle handling can be achieved through the use of external magnetic or electric fields to position certain particles or physical interaction between certain particles and manipulation instrument (Castillo et al. 2009). Microgripper technology has been demonstrated using microfabricated electrostatically actuated grippers for a ‘‘pick-and-place’’ technique to move a single nanowire (Kristian et al. 2006). Microelectromechanical systems (MEMS) with force feedback systems have also been developed to handle nanosized particles (Beyeler et al. 2007). Microfluidic systems are often utilized to handle small particles. These systems provide advantages that can include an increase in automation and a reduction in both the reagent and sample used (Vilkner et al. 2004). Microfluidic systems can take the form of enclosed fluidic chambers fabricated in processes such as etching in silicon and sealing with glass (Beyeler et al. 2007), hot embossing in plastics (Gattikera et al. 2008), molding in PDMS (Xia and Whitesides 1998), or creating open fluidic systems. The latter involves fluids being bound by air/liquid interfaces, as is the case in open channels (Chen et al. 2008; Davey and Neild 2011) or droplets on a surface (Wixforth 2003; Oberti et al. 2009a). Simultaneous manipulation of nanoparticles in solution within microfluidic systems has important

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analytical and sensitivity benefits, as it allows concentration and collection at sensing locations. There are a range of methods available, including optical tweezers (Ashkin 1970) and dielectrophoresis (Beskok 2010). When applying these techniques to droplets, in the case of optical tweezers, the diffraction which occurs at the air/liquid interface will complicate the production of the highly focused laser beams required. For dielectrophoresis, the force field produced diminishes rapidly as distance from the electrodes is increased. In channels a solution which is not directly available for a droplet is used, for ensuring a low channel height and hence minimal particle distance from electrodes. In contrast, implementing mechanical vibration, as performed in this work, in droplets is no more challenging than a similar implementation with microfluidic channels. A good example of the ease of applying vibration to droplets comes in the form of using acoustic radiation forces. These forces are usually exploited by exciting a resonant pressure field in the fluid at an ultrasonic frequency. In this scenario a time averaged force exists on suspended particles which causes collection at either the pressure nodes or antinodes, depending on particle and fluid properties (Petersson et al. 2005). These forces have been used to collect particles in lines (Neild et al. 2007a; Wiklund et al. 2006), grids of points (Neild et al. 2007b), sort particles (Laurell et al. 2007; Nilsson et al. 2004), and force particles onto a surface (Hawkes et al. 2004; Martin et al. 2005). When applying these methods to particle manipulation in a droplet (Oberti et al. 2009a) the authors used the same transducer arrangement as had previously been used for particle manipulation in an enclosed chamber (Hagsater et al. 2007). Naturally the geometry of the fluid affected the patterns formed and the resonant frequencies, but the method of implementation required no alteration. The use of acoustic radiation forces to manipulate nanosized particles has been very limited. Single and multi-walled carbon nanotubes have been aligned into positions through the uses of surface acoustic waves (Seemann et al. 2006). In addition, it has been demonstrated that acoustic radiation forces can be used to position nanosized diamond particles into concentric nodal patterns (Raeymaekers et al. 2011). A second option in mechanical vibration is the use of low frequency actuation. Again when applied to droplets the implementation is very similar to channels (Oberti et al. 2009b). However, the large air–liquid

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J Nanopart Res (2012) 14:1223

interface allows the generation of capillary waves and associated effects related to fluctuation in the contact angle. Large surface deflections can cause the wetted area (on the solid surface) to increase or decrease in size as the advancing or receding contact angles are exceeded, respectively. Hence low frequency actuation has been used to move droplets up an inclined surface with vertical vibration (Brunet et al. 2007) and across horizontal surfaces using asymmetric lateral vibration (Daniel et al. 2005; Dong et al. 2006). A similar mechanism is at play when low frequency actuation is used to spread fluid droplets (Whitehill et al. 2011, 2012), which under high amplitudes behave in a nature similar to droplets impacting on a substrate (Rioboo et al. 2006, 2009). This spreading technique has been utilized to fill multiple microplate wells simultaneously (Chong et al. 2012). In terms of particle manipulation, these surface oscillations are associated with strong fluid velocity fields within the droplet, and effect which has recently been used for collection of particles greater than 40 lm in diameter (Whitehill et al. 2010). In this work we present a hypothesis predicting how to reduce this minimum particle size for collection and proceed to demonstrate the collection of 190 nm diameter particles. Experimental setup The experimental setup is illustrated in Fig. 1. Three different sizes of particles were used for this investigation. These were polystyrene nanospheres, manufactured by Bang Laboratories Inc. with sizes of 190 nm (FS02F), 500 nm (PS03 N), and 1010 nm (PS04 N). Polystyrene nanoparticles were used due to their widespread availability and their density being relatively close to that of water. The nanoparticles were diluted into de-ionized water. The size of the de-ionized water droplet deposited onto the substrate was also varied between 1.9 and 5.5 lL. This change in volume would strongly affect the ensuing contact angle of the droplet. This droplet was deposited onto well on a glass slide. This well was defined by a Teflon coating, creating multiple 4 mm diameter circular wells on the glass surface (ProSciTech G352104-W). The slide was positioned on an aluminum slide holder. This holder was attached to the top of an electromagnetic shaker (Bruël and Kjaer, LDS V201). The shaker was driven by a power amplifier (Bruël and Kjaer, LDS PA 25E) via a signal generator (Stanford


Fig. 1 The experimental setup is depicted; a glass slide with hydrophilic circular areas surrounded by a Teflon coating is mounted on an electromagnetic shaker

Research SDR 345), with the direction of motion being normal to the surface of the slide. Illumination of the experiment was provided via a fiber optic light source (Edmund Optics, model MI-150). A charged-coupled device camera (Hitachi, KP-D20AU) fitted with a magnification lens (InfiniVar Video Microscope, Infinity Photo-Optical Co.) was mounted above the setup to visually record the experiment at 29 frames per second directly onto a computer using a video capture card (Leadtek WinFast VC100 U). For the experiment involving high speed video capture this setup was replaced with a high speed camera (Fastec Imaging Troubleshooter TS1000ME) at 1,000 fps. These were registered directly onto the internal hard drive of the camera, producing an uncompressed video file, which was subsequently transferred to a computer. Mechanism of clustering The goal of this work is to immensely decrease the size of particles which can be collected using low frequency vibration from the microscale to the nanoscale.

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To this end it is productive to examine the previous study on collection of particles using low frequency vibration (Whitehill et al. 2010). In that work it was found that two types of forces acted on the particles. In order to distinguish between them it is necessary to consider non-linear effects in the droplet vibration. When a droplet is excited harmonically it will cause a harmonic motion of both the fluid/air interface and the fluid within the droplet. These are both first order effects as they occur sinusoidally. In addition to these first order harmonic responses, there will be a steady state acoustic streaming flow. This arises from the time average of the second order terms in the Navier– Stokes equation being non-zero (the integration over a cycle of terms including, for example, sin2 ðxtÞ are non-zero). This second order effect is not sinusoidal in nature; rather a steady swirling of fluid ensues. From each of these scenarios, forces develop which act on the particles. The acoustic streaming induces drag forces which act to cause the particles to follow the streaming flow, resulting in a circulating motion. In contrast, the forces which collect the particles in stable locations result from the first order flow field. We can be sure of this as it was possible (in (Whitehill et al. 2010)) to observe the particles moving at the oscillation frequency. The collection forces are non-linear in character (there is a collection over multiple cycles) but they arise directly from the first order fluid motion. A similar scenario exists at ultrasonic frequencies. In that regime acoustic radiation forces are a non-linear force arising from the first order vibration terms and, under the right conditions, act to collect particles in stable patterns. Acoustic streaming acts to swirl the particles around as the drag forces cause them to follow the second order flow field. It has been shown that as the acoustic radiation force is proportional to the particle radius cubed and the drag arising from streaming is proportional to the radius, smaller particles become relatively more affected by streaming (Hagsater et al. 2007). This result is similar to that at low frequencies, in that there is a minimum particle size which can be collected, below which streaming dominates. Returning to low frequency actuation, the collecting force is less well characterized. In part this is due to the complexity of modeling the surface tension dominated interface of a vibrating droplet. The harmonic motion displayed by the particles during collection, points to a form of multiple pass hydrodynamic focusing (Vilkner et al. 2004). In order to collect nanosized

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particles, the effects of streaming must be reduced preferentially over the effects of the collection forces. We can deduce that the second order flow must be significantly reduced in comparison to the first order flow. The steady state second order streaming flow will be governed by a no-slip boundary condition at the solid surface interface and a no shear stress condition at the fluid air interface. The shear stresses which need to be overcome, due to that no-slip condition, are given by s ¼ l du dy, where l is the dynamic viscosity, u is the flow speed, and y the height above the solid surface. This being the case, for a given force field driving the flow, a reduction of the height of the droplet will reduce the maximum flow obtained. For the first order oscillations a key parameter describing the flow characteristics is the Stokes boundary layer thickness, d, given by: rffiffiffiffiffi 2v d ¼ 2p ð1Þ x where m is the kinematic viscosity and x is the angular frequency. Again this is relevant with respect to the no-slip boundary condition at the solid surface. This expression comes from a simpler flow than that in a vibrating droplet; however, it can be seen that it predicts a boundary layer of approximately 236 lm at 225 Hz. Hence the first order flow, above that the distance below droplets surface, is not limited by the no-slip condition at the solid interface. By considering the effect of the boundary conditions on the first and the second order flow a number of parameters can be defined. It becomes clear that a reduction in fluid height will lessen the second order flow, but the effect on the first order flow will be much less significant due to the relatively thin boundary layer. This discussion indicates that a reduction in fluid height should be effective in reducing the size of particles which can be collected. The main focus of this article is the demonstration that this is indeed correct, and examining to what degree we can reduce the minimum particle size which can be collected. Droplet resonance The actuation frequency for shallow capillary waves on the surface of a fluid is at the fluid frequency (Noblin et al. 2004). Noblin et al. developed a theory

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for the resonant vibrational modes of a sessile drop. Resonance is vital for the ability to accumulate particles into specific locations within a drop. This model was achieved by establishing a relationship between the dispersion for capillary waves in an infinitely deep fluid bath with the half number of integer wavelengths that can fit around the arc length of the meridian curve. The wave properties of dispersion for the surface of a capillary droplet can be used to estimate the frequency of oscillation. With the frequency for a sessile droplet found as: f ¼

2pc qks

1=2 ð2Þ

where f is the frequency of oscillation, c is surface tension of the fluid, q is the density of the fluid, and k is the pseudo wavelength of the wave. The pseudo wavelength is defined as the mean distance between two consecutive nodes along the droplet arc. This ‘‘k’’ mode resonant modal type is defined by Noblin et al. (2004) as: p ¼ Rh ¼

kkk 2

ð3Þ

where h is the contact angle of the drop, R is the radius of curvature of the droplet, k is the mode number of the wave, and p is the arc length of the meridian curve from the center to the edge of the drop at equilibrium. The droplet shape can be assumed to be a spherical cap if it satisfies the following equation (Sharp et al. 2011): V 0:1L3cap

ð4Þ

With Lcap defined as the capillary length for the droplet (*2.7 mm for water). All volumes used in this research are smaller than the critical volume as defined by Eq. (4) and so the droplets can be assumed to be spherical caps prior to excitation. Hence the volume of the cap can be approximated by: V¼

m pR3 bðhÞ ¼ q 3

ð5Þ

With bðhÞ ¼ cos3 ðhÞ 3 cosðhÞ þ 2 and m as the mass of the drop. Hence combining Eqs. (2), (3), and (5) we have an approximation for the kth modal shape: 1=2 p k3 cbðhÞ fk ¼ ð6Þ 2 3mh3


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This theoretical approximation was used to find resonant conditions for the droplet. For a 3 lL drop with a kth modal shape of four it was found that the resultant vibrational frequency was 225.1 Hz. This estimation was confirmed using a high speed camera. The theoretical estimation provides a high correlation with the experimental data with minimal error, even when the estimation neglects damping in the droplet system and uses assumed values for the environmental conditions in the calculation of the frequency.

Results and discussion The location of the nanoparticles relative to the wave formation and subsequent fluid flow was investigated first. Nanoparticles form concentric rings underneath the tangent to the nodal locations on the surface after being influenced by low frequency oscillation. High speed analysis shows that the wave shape of the droplet follows a resonance condition first described by Noblin et al. (2004). In this work when a droplet is excited at the kth mode resonant modal type the contact line of the drop becomes mobile with an antinode of the fluid motion forming at this location. In the case of a hydrophilic well surrounded by a hydrophobic surface, it would be expected that the type of pinning arising at the transition between these surface chemistries, would cause a static contact line at the edge of the well. Despite this, it can be seen from Fig. 2 that this is not the case; there is indeed an antinode at the contact line. In considering that the ‘‘real world’’ printed Teflon surfaces have imperfect nature, this surface roughness obfuscates the transition between the hydrophilic and hydrophobic surface and as a consequence reduces the pinning boundary condition. This allows a certain amount of freedom around the contact line, with the fluid wave motion forming an antinode at the contact line of the droplet. The subsequent fluid motion causes the nanoparticles to be positioned at a tangent to the free surface beneath the wave nodes of the drop. Three different sized nanoparticles ring formations are shown in Fig. 3a–c. The particles are influenced by a hydrodynamic focusing phenomenon, in which particle will be pushed back and forth in the flow of the fluid. Due to the fluid motion oscillating directions in each actuation cycle, a time averaged effect will cause the particle to aggregate toward the nodal locations of the

Fig. 2 The upper image shows a horizontal view of the vibrating droplet taken with a high speed camera, the superimposed line is the profile of the droplet one half a cycle later (taken from a similar high speed image). The displacement nodes have been marked with circles for clarity. The lower image was taken from the vertical position, and shows particles collected in circles (a quarter of which are visible). A line dropped normally to the tangent of the fluid profile at the locations of the nodes, corresponds to the location of particle collection

fluid flow. This creates ring formations of particles on the surface of the substrate. The formation of the rings can clearly be seen for all particle sizes, though one of the key differences is the width of the rings. Figure 3d shows the intensity measured from the images across the droplet. The spikes correspond to collection of particles (which reflect the illumination light). The larger particles can be seen to form thinner rings whereas the smaller particle formation is wider and not clearly defined. The ring widths (measured as the full-width-halfmaximum) for the 1,010, 500, and 190 nm, were averaged to be 38.8, 46.7, and 107.5 lm, respectively. This would suggest that a size dependent mechanism is at play with this manipulation technique, as expected.

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Fig. 3 Concentric rings of a 190 nm, b 500 nm, and c 1010 nm diameter particles are formed beneath the nodes of the surface wave. d The intensity of the images show collection for the three cases which can be compared with the intensity prior to excitation (unperturbed)

Next we must consider the hypothesis that a reduced fluid height can allow smaller particles to be handled. In the previous work a volume of 50 lL was placed in a 10 mm diameter by 0.25 mm deep well. The corresponding height of the droplet was 1,050 lm. Here the volume is reduced to 3 lL, and is placed in a well of 4 mm diameter. Assuming that the fluid forms an approximately spherical cap, we can estimate the height of the drop using the following equation: 1 V ¼ ph 3r 2 þ h2 6

ð7Þ

where V is the volume of the drop, h is the height of the drop, and r is the radius of droplet. This corresponds to the maximum height of a 3 lL droplet being 465 lm. Particle clustering requires the condition of a relatively shallow droplet shape, to reduce the acoustic microstreaming that can affect the particles. In order to investigate variation of fluid height on the effects of focusing nanoparticles we used the natural droplet evaporation to vary this parameter. A droplet in normal atmospheric conditions will evaporate at a rate that is dependent on the humidity and temperature as

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well as the surface area of the drop. Through the use of this we can study the height effects on the particle behavior. The rate of evaporation for the drop is crucial in maintaining resonance within the fluid. When the droplet evaporates the resultant profile length will decrease. This will cause a frequency shift required for the droplet to maintain this resonant condition. The linking between height and resonance means that the two factors are not studied in isolation, but the experiment demonstrates the effect of reduced fluid height on particle manipulation capabilities. The change in the peak height of the droplet for the largest to the smallest experimental volume, 5.5 and 1.9 lL, respectively, results in a 74 % reduction in height; while at the same time the frequency variation to maintain resonance increases only by 15 %, to remain at kth modal number of four. A 5.5 lL droplet containing 500 nm particles was placed on the experimental slides (shown in Fig. 4a). At this volume the droplet has a peak depth of 830 lm and a contact angle of 45°. The time period for the experiment was 600 s and volume varied from 5.5 lL down to 1.9 lL. Upon actuation acoustic streaming


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Fig. 4 Four different characteristics behaviors of an evaporating droplet; before actuation a the droplet was 5.5 lL (203 Hz resonance), 200 s after actuation commence b streaming is visible in four distinct three dimensional cells (4.4 lL), at 400 s

c rings have begun to be formed (3.2 lL), and at 600 s d strong ring formation starts being disrupted by the final stages of drop evaporation (1.9 lL)

dominates the movement of nanoparticles. Nanoparticles travel in four separate three-dimensional circulation cells with no formation of clear rings. As the depth of the droplet reduces streaming effect and the hydrodynamic effects are in a transition zone, with neither phenomena dominating the other. This region is shown in Fig. 4b with the motion in the still image indicated by the red arrows on the figure. In this figure the droplet depth is approximately 620 lm and has a contact angle of 34°. Hydrodynamic focusing begins to overpower the microstreaming mechanism when the droplet volume reduces further. Figure 4c shows concentric ring formations beginning to distinctly form when the depth was 510 lm with a contact angle of 28.6°. Nanoparticles continue to remain within these nodal regions until the cessation of the experiment, experiencing a very slight shift in position due to the small movement of the nodal locations on the drop.

The final particle locations are shown in Fig. 4d, when the contact angle and depth were merely 17.5° and 300 lm, respectively. Manipulation of nanoparticles becomes quite difficult in extremely low volume droplets. This is due to the presence of minimal fluid movement around the nodal locations. The size of the Stokes boundary layer of the drop approaches the majority of the fluid height. At the smallest volume used (1.9 lL) the peak depth was 300 lm while the boundary layer thickness was 232 lm. This means that as the volume decreases toward the limit, the potential for the strong oscillating fluid motion required for hydrodynamic focusing significantly reduces. The creation of these concentric rings occurs over thousands of oscillation cycles. With the hypothesis that the reduced height of the fluid aids the collection of the particles supported (within limit), we now can examine the nature of the collection of nanoparticles

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Fig. 5 Time evolution of the formation of 500 nm particles, collection takes place over a period of 200 s

in terms of the time of collection and the effect of excitation on amplitude. Figure 5a–d shows the images from a video of 500 nm particles forming concentric rings under the nodes over a period of time. The video of this experiment was analyzed using MATLAB. In order to produce the intensity across the droplet centerline, the first and the last frames were extracted from the video. The first frame was then thresholded using the mean value of the intensity as the threshold value. This allowed the edges of the droplet to be found and centerline identified. The intensity plots for the first and the last frames were simply the intensity values across this centerline. The plot of this is shown in Fig. 5e, in which the original droplet intensity (blue) demonstrates the smooth profile distribution of the nanoparticles due to the variation in fluid height across the droplet. In contrast, the final droplet intensity profile (red) exhibits distinct peaks of concentrated particles at the nodal locations of the waves. This is shown through a drop in intensity, excluding the peaks, over the whole droplet. This indicates that while not all particles have aggregated toward the nodal locations, a significant number have. The growth over time of the inner and outer rings is plotted in Fig. 5f. This shows the difference between

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intensities of each successive frame and the first frame. The outer ring (blue) develops a stronger, more clearly defined ring in comparison to the inner ring (red). The plots of the intensity differences at the peak locations were smoothed using a robust local regression with weighted linear least squares and a second degree polynomial model. In any microscale acoustofluidic system the amplitude of oscillation must also be considered as this has the potential to drastically alter the behavior of the fluid, and the fluid flow profile. We analyze different amplitude conditions and their effect on the formation of particles. Figure 6 shows the range of distinct particle manipulation behaviors at different oscillation accelerations. The first region (Fig. 6a) is when the acceleration is minimal (2 g) and the mechanism for hydrodynamic focusing is too weak to cause any particle collection. The particles remain dispersed throughout the droplet regardless of the time they are exposed to excitation. The second region occurs between accelerations of approximately 2–4.4 g. In this excitation range, particles form distinct concentric rings on the surface of the substrate underneath the nodal locations of the surface wave. Particles experience ideal conditions for manipulation by this hydrodynamic focusing, as


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Fig. 6 The images show particle behavior at a range of different amplitudes of excitation, in all cases the drop is 3 lL, the excitation is at 225 Hz and 500 nm particles are used, the images, taken after 150 s of actuation, correspond to actuation

accelerations of a 0.8 g at which no particle collection is observed, b 2.9 g at which distinct collection occurs, c 5.3 g at which collection lessens and finally d 6.7 g at which streaming prevents collection from being effective

shown in Fig. 6b. Upon further increase in the acceleration of the droplet system, between approximately 4.4 and 6.3 g, this focusing mechanism becomes less effective (Fig. 6c). Before breaking down, under even higher accelerations (larger than 6.6 g), streaming effects begin to overpower the hydrodynamic focusing forces, greatly affecting the particle focusing performance (Fig. 6d). Particles that are subjected to acceleration within the second region (2–4.4 g) will create rings within a similar time frame. This shows that the rate at which particle focusing occurs is only weakly dependent on the oscillation acceleration within this region. The number of vibration cycles that the particles will be influenced by will be the same within this range of accelerations. The motion of the fluid will create the same nodal shapes, although the amplitude of the waves will differ. Taken in the context of the effect of

changing the height of the droplet as seen in Fig. 4, the change in amplitude will affect the height at which the transition between behavior patterns occur. Previously it has been shown that a droplet experiencing vibrations can move between different types of oscillation modes as the amplitude is increased and that the oscillation modes were at half the excitation frequency (Whitehill et al. 2010). However, in the comparatively small droplet volume used in this work, the mode of oscillation was at the actuation frequency and the nodal shapes did not change within the range of experimental conditions used. This was confirmed using a high speed camera, the data from which was examined to see if a change in vibration mode shape occurred over the amplitude range used. A microscale acoustofluidic system’s ability to manipulate nanosized particles is highly dependent on a large range of parameters. Hydrodynamic focusing

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within a droplet is no different, requiring a balancing act to achieve the desired result. The conditions must be within a defined range with a number of parameters strongly influencing the ability to collect particles. These parameters include the droplet height, actuation frequency, and amplitude. If the droplet height is too low the particles will be caught in the boundary layer of the drop and collection will be inhibited. If the droplet is too high, streaming effects overpower the hydrodynamic focusing effects. Resonance must also be maintained to provide strong nodal locations for the particles to accumulate. The particles are also affected by large actuation amplitudes as this aids the onset of acoustic streaming within the droplet. On the other end of the spectrum, if the actuation amplitude is too small, the droplet will have insufficient fluid movement for this focusing effect. Conclusions The examination of forcing mechanisms which act on particles suspended within droplets, when vibrated at low frequencies, has shown that particles can be manipulated down to a size of 190 nm. This represents a massive reduction in size from the previous work conducted using these mechanisms. By lowering the height of the droplet the effect of acoustic streaming is lessened to a greater degree than the effect of the first order oscillation of the particles. The effect of lowered height has been demonstrated by allowing a droplet to evaporate while maintaining resonant excitation; in this scenario a critical height is reached, below which collection becomes dominant over swirling. A second parameter which strongly affects collection behavior is the amplitude of excitation; since excessively increased amplitude is detrimental in the collection process, this parameter will affect the height value at which the transition between swirling and collection occurs. Acknowledgments The authors were supported in this work by the Australian Research Council through a Discovery Grant: DP110104010.

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