Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

This article reviews acoustic microfluidics: the use of acoustic fields, principally ultrasonics, for application in microfluidics. Although acoustics is a classical field, its promising, and indeed perplexing, capabilities in powerfully manipulating both fluids and particles within those fluids on the microscale to nanoscale has revived interest in it. The bewildering state of the literature and ample jargon from decades of research is reorganized and presented in the context of models derived from first principles. This hopefully will make the area accessible for researchers with experience in materials science, fluid mechanics, or dynamics. The abundance of interesting phenomena arising from nonlinear interactions in ultrasound that easily appear at these small scales is considered, especially in surface acoustic wave devices that are simple to fabricate with planar lithography techniques common in microfluidics, along with the many applications in microfluidics and nanofluidics that appear through the literature.

Figure 1

The range of acoustic wave excitation at the close of the 20th century. The upper limit of this range, a few THz, has drawn the interest of many researchers in the past decade. The generation, analysis (332), and creative use (10) of THz-order acoustic waves has become almost routine since this figure was originally published. From 431.

Figure 2

Medical treatments and research in medical ultrasound span the 20 kHz–10 MHz range. The range of treatments encompasses both the destructive capabilities of high-power ultrasound in lithotripsy, thrombolysis, and drug delivery, and the sensitivity of ultrasound transduction in imaging. From 284.

Figure 3

Piezoelectric memory: PZT polycrystalline nanostructures annealed for 1 h at $650°\mathrm{C}$. From 4.

Figure 4

The simplicity of (a) the first interdigital electrode (from 432) remains attractive for many applications despite the usefulness of more complex designs such as (b) the SPUDT which directs most of the acoustic wave energy in a single direction. (c) Circulation structure used to return acoustic energy back to the supply transducer (from 13). (d) Tapered designs for focusing acoustic energy.

Figure 5

Wave solutions representing (a) longitudinal, (b) quasitransverse, $z$-polarized shear, and (c) quasitransverse, $y$-polarized shear acoustic wave propagation along the positive $x$ direction, without coupling to the electric field. Most surface waves are a combination of these motions; for example, Rayleigh and Sezawa waves combine (a) longitudinal and (b) transverse, $z$-polarized shear motion, while Love and SH-SAW waves combine (a) longitudinal and (c) transverse, $y$-polarized shear motion.

Figure 6

The many forms of acoustic wave transmission as indicated in an incomplete hierarchical list. Among those shown, the boxed items are most likely to appear in acoustic microfluidics.

Figure 7

(a) A $100\mu \ell$ water drop exhibits (b) axisymmetric waves followed by a breakage of azimuthal symmetry and the formation of (c)–(e) azimuthal-radial waves that eventually result in (e) drop ejection and (f) complete drop breakup as the amplitude of vibration is increased. From 413.

Figure 8

Collection of (a) melamine (hydrophilic) and (b) polystyrene (hydrophobic) particles. Some of the melamine particles were left alongside the drop track, while some of the polystyrene particles were dropped out across the width of the drop in a regimented monolayer ring formation. From 391.

Figure 9

The patterning of fluid thin films into regularly spaced “dots” using mixed-mode (standing and traveling-wave) Rayleigh SAW. Here $a$ and $b$ are a few tens to a few hundred micrometers, depending on the frequency of the SAW device. From 5.

Figure 10

Illustration of a flexural plate wave micropump structure showing (a) straight and (b) curved interdigital electrodes that are very similar to those used in surface acoustic wave devices. The silicon nitride is the bottom layer in the (a) cross section inset. From 301.

Figure 11

By selectively driving the external fluid cavities (not shown) at resonance, 224 were able to pump fluids at about 100 Pa through their microfluidics device as desired among the four input channels. From 224.

Figure 12

The SAW device (a) combined with a PDMS structure that has an open-ended channel. Turning on (b) the Rayleigh SAW gives rise to a fluid flow through the channel; the fluid progresses rapidly through the channel. Since PDMS is hydrophobic minutes after its bonding to a substrate (114), this flow is into a hydrophobic microchannel. From 52.

Figure 13

As the width of the $100\mathrm{\text{−}}\mu \mathrm{m}$ deep, 2-cm long laser-cut channel in LN is increased from (a) $25\mu \mathrm{m}$ to (b) $75\mu \mathrm{m}$, (c) $150\mu \mathrm{m}$ and (d) $280\mu \mathrm{m}$, the fluid flow within goes from a uniform $2\mathrm{cm}/\mathrm{s}$ flow to a strongly mixing flow. By reducing the input power, particles suspended in the fluid can be made to collect along (e) one or (f) more lines as a consequence of cross-channel standing waves, a topic covered later in this review. The (g) channel cut is parallel to the $x$ axis in the 128YX LN substrate. From 392.

Figure 14

Experimental images showing the transition from (a) drop vibration to (b) jetting, (c) pinch-off of a single droplet, and (d), (e) jet breakup to form multiple droplets by increasing the jet Weber number ${\mathrm{We}}_j\equiv {\rho }_f{U}_j^2{R}_j/\sigma$, where $R_j$ and $U_j$ are the radius and velocity of the jet, respectively, and $\sigma$ is the surface tension. From 393.

Figure 15

Comparison of the predicted jet velocity to the measured jet velocity for various fluids, drop sizes, and power inputs in SAW-induced jetting. From 393.

Figure 16

A BSA-encapsulated PCL polymer microparticle formed using SAW atomization. From 7.

Figure 17

(a) The SAW is generated on the substrate and (b) interacts with a fluid drop, causing the drop to deform into an asymmetric conic leaning approximately at the Rayleigh angle, ${\theta }_r\equiv {sin}^{-1}(c_f/c_{\mathrm{SAW}})$, where $c_f$ and $c_{\mathrm{SAW}}$ are the speed of sound in the fluid and of the SAW in the substrate, respectively. Although indicated here with a simple monofrequency capillary wave, the wave is actually far more complex. The Rayleigh SAW’s retrograde particle motion and the exponential decay (c) are indicated, as are the effects of acoustic irradiation which causes drop deformation through first-order effects on the time scale of the acoustic wave and acoustic streaming-driven bulk fluid recirculation on the hydrodynamic time scale. From 322.

Figure 18

Results from a fast Fourier transform frequency sweep using the scanning laser Doppler vibrometer showing the frequency at which the capillary wave on the free surface of the drop is excited. (a) Drop with finite thickness of $\sim {10}^4\mathrm{m}$: The resonant frequency of the capillary wave is $\sim 10\mathrm{kHz}$. (b) Thin film of thickness ${10}^6\mathrm{m}$: The resonant frequency of the capillary wave is $\sim 20\mathrm{MHz}$, the exciting frequency. From 322.

Figure 19

The behavior of a sessile drop of a colloidal suspension containing 500 nm fluorescent polystyrene particles atop a Rayleigh SAW device dramatically varies depending on the input power. At relatively low powers (region A) the particles tend to collect in a pattern defined by the wavelength of the SAW in the substrate. As the power is increased there is a narrow region (region B) where the particles collect in distinct spots separated by a distance corresponding to the capillary wavelength on the fluid’s surface, followed by a chaotic rotation and collection phenomena in region C and bulk rotation and particle collection in region D reminiscent of 365. From 238.

Figure 20

Results of the computation of the transmission of acoustic waves into a sessile drop after (a), (b) 10, (c), (d) 30, and (e), (f) 50 periods, using 128YX LN driven by a straight, standard interdigital electrode (IDT) with 30 electrode finger pairs at a resonant frequency of 20 MHz. The amplitude of the electric field input into the IDT was adjusted to deliver a SAW amplitude on the free surface near the drop edge of 1.2 nm. The black lines represent the greatly exaggerated deformation height of the free surface. From 394.

Figure 21

Particles $6\mu \mathrm{m}$ in diameter collect along lines upon the surface of a $3\mu \ell$ sessile drop (height $\sim$ length), using 30 MHz standing-wave Rayleigh SAW generated by two focusing, standing-wave ultrasonic interdigital transducers placed at the left and right of the drop in a configuration reported by 393. The distance between two adjacent collection lines is $32\pm 4\mu \mathrm{m}$, roughly the same as one-half the wavelength of the acoustic wave in water; the difference is believed to be due to the curvature of the fluid drop, as illustrated by the (b) computational result showing the free surface profile as a black line and the pressure amplitude in the contour plot. From 394.

Figure 22

The size of the particles affects the ability to collect them via audible acoustic irradiation, as shown by oscillation at 60.59 Hz, for particle diameters of (a) $10–30\mu \mathrm{m}$, (b) $42\mu \mathrm{m}$, (c) $60\mu \mathrm{m}$, and (d) $116\mu \mathrm{m}$. From 433.

Figure 23

Using a setup similar to 236 with a $1.5\mu \ell$ water drop set atop a 20 MHz LN SAW device. (a) Polystyrene particles 15 and $20\mu \mathrm{m}$ in diameter concentrate at the center of the drop while 25 and $31\mu \mathrm{m}$ diameter particles concentrate at the periphery. The smaller two sets of particles sit below the crossover diameter of about $22\mu \mathrm{m}$ where (b) the force due to drag $F_D$ is equal to the direct acoustic radiation force $F_{\mathrm{AR}}$. Their trajectories are mainly defined by the acoustic streaming-driven Stokes drag forces, and so their behavior is similar to what is seen by Li et al.. The larger two sets of particles, on the other hand, are propelled to the side of the droplet and held there by the direct acoustic forces which dominate over the acoustic streaming. From 338.

Figure 24

As the excitation frequency is raised, distinct patterns appear as defined by the pinning of the particles within a sessile drop; the drop is $\sim 2\mathrm{mm}$ in diameter and 1 mm in height. From 307.

Figure 25

An indication of whether particles suspended in a sessile drop will concentrate via the vortex flow phenomenon and shear migration. The filled marks indicate dispersion of the particles, while unfilled marks indicate concentration. The transition to concentration occurs as $\chi$ grows beyond the critical value of ${\chi }_c=2$ for $10\mu \mathrm{m}$ polystyrene particles in water. From 236.

Figure 26

(a) Schematic of the dual-port SAW device from 203, with drop and (b) $7–9\mu \mathrm{m}$ diameter, $30\mu \mathrm{m}$ long Cr tubes in a fluid thin film formed by a glass cover slip, prior to applying the SAW. Applying 13 dBm at 30 MHz to the two IDTs (c) moves the tubes to nodes of the standing SAW in the fluid, and (d) aligns the tubes to the SAW’s propagation direction. Increasing the concentration of the tubes in the fluid can (e) cause them to chain together, bridging the nodes. From 203.

Figure 27

The magnitude of the fluid velocity in a $280\mu \mathrm{m}$ wide channel (see Fig. 13) viewed from the top down with $x_1$ from left to right, due to 20 MHz acoustic waves refracting into the water in the channel from the LN after (a) 4 and (b) 10 cycles of the SAW. This model incorporates bidirectional coupling between the piezoelectric LN and the fluid, and (c) the cross-channel standing wave with its pressure nodal lines causes (d) particles to collect along six distinct lines across the width of the channel, directly corresponding (e) to the pressure node lines. From 392.

Figure 28

Schematic of how one could use phononic crystal structures to steer acoustic waves as desired. From 310.

Figure 29

(a) The microgroove acts to carry the acoustic wave along its length illustrated by the (b) pressure measurement which clearly shows the isolation of the acoustic wave to the groove compared to a (c) substrate without the groove. Looking (d) end on, the distribution of the acoustic energy is shown to be mainly in the walls of the groove. From 244.

Figure 30

Channel fabricated directly in LN using (a) a FIB system; the channel is 510 nm wide, 500 nm deep, and runs for 1 cm along the $X$ axis of the material. Using EBL, we have been machining IDTs onto the surface for GHz-order SAW using (b) acrylic patterned for use in a liftoff technique.