Acoustic streaming vortices enable contactless, digital control of droplets

Abstract

Advances in lab-on-a-chip technologies are driven by the pursuit of programmable microscale bioreactors or fluidic processors that mimic electronic functionality, scalability, and convenience. However, few fluidic mechanisms allow for basic logic operations on rewritable fluidic paths due to cross-contamination, which leads to random interference between "fluidic bits" or droplets. Here, we introduce a mechanism that allows for contact-free gating of individual droplets based on the scalable features of acoustic streaming vortices (ASVs). By shifting the hydrodynamic equilibrium positions inside interconnected ASVs with multitonal electrical signals, different functions such as controlling the routing and gating of droplets on rewritable fluidic paths are demonstrated with minimal biochemical cross-contamination. Electrical control of this ASV-based mechanism allows for unidirectional routing and active gating behaviors, which can potentially be scaled to functional fluidic processors that can regulate the flow of droplets in a manner similar to the current in transistor arrays.

Fig. 1. ASV-based droplet manipulation.

(A) Schematic showing a typical droplet processing unit. The droplets (i.e., red and blue spheres) over the transducers are guided into the center between the barrel-like ASVs (labeled as Vortices) following the recirculating inflow. These droplets are unidirectionally routed along the linear array of IDTs by shifting the sequence of working frequencies. The portions of the IDTs shaded in purple (e.g., IDT_i) indicate that they are being excited by a high-amplitude signal with frequency f_i. (B) The photo shows a bifurcated device with a particle floating above the transparent carrier oil layer. (C) The general control schematic for the droplet processing unit. The unit is composed of K + M + N interconnected IDTs [denoted as ${\text{IDT}}_k^I$, ${\text{IDT}}_m^L$, and ${\text{IDT}}_n^R$ (k = 1,2, …, K; m = 1,2, …, M; n = 1,2, …, N)] with tuned working frequencies. Multitonal signals (i.e., S_IL or S_IR) encoded with a series of different frequencies, amplitudes, durations, and initiation times (i.e., ${[f_m^L,A_m^L,T_m^L,t_m^L]}_{K+M}$ or ${[f_n^R,A_n^R,T_n^R,t_n^R]}_{K+N}$) are the excitation signals into the droplet processing unit and can direct the droplet from ${\text{IDT}}_1^I$ to the left or right port, respectively. The blue shaded area indicates the resulting virtual bifurcated channel for droplet translation. Photo credits: Peiran Zhang, Duke University.

Fig. 2. Mechanism of droplet actuation via ASVs.

(A) The xy-plane composite image of particle trajectories generated from stacked, bottom-view images of particles near one flank of a transducer when excited by a low-amplitude signal near the surface of the oil (z = 1.04 mm). (B) xy plane of particle trajectories generated from the stacked, bottom-view images of particles near one flank of a transducer when excited by a high-amplitude signal beneath the surface of the oil (z = 0.77 mm). The blue arrows indicate the trajectories of fluid being pumped outward. Red arrows indicate the trajectories of fluid pumping inward. White arrows indicate the trajectories of recirculating flows. The shaded red areas indicate the location of the actuated transducers. (C) Numerical simulation results showing the acoustic streaming pattern in the xy plane (z = 1.04 mm) with a low-amplitude excitation signal. (D) Numerical simulation results showing the acoustic streaming pattern in the xy plane (z = 0.77 mm) with a high-amplitude excitation signal. Simulation results showing the acoustic streaming pattern in the xz plane (y = 0 mm) with (E) a low- and (F) a high-amplitude excitation signal. The black asterisks indicate the hydrodynamic equilibrium positions. Scale bars, 500 μm.

Fig. 3. PIV analysis with increasing excitation amplitude.

(A) The particle velocity distributions around the ASVs (z = 1.04 mm; on the oil surface) as the excitation voltage is increased. A.U., arbitrary units. (B) The velocity distribution of the particles, escaping from the transducer along x axis (i.e., x velocity) over the flanks of the transducer (z = 1.04 mm; on the oil surface), shifts toward the right side of the graph as the excitation voltage increases. (C) The velocity distribution of the particles moving away from the transducer along the y axis (i.e., y velocity) near the aperture of the transducer (z = 0.77 mm; inside the carrier oil) also shifts to the right in the graph as the excitation voltage increases. The dashed line indicates zero particle velocity (i.e., static particles). Note that the particle trajectories of the higher speed inflow are obscured by those of the overlapping channel vortices. (B) and (C) share the same legend. The normalized particle count is derived through PIV analysis on small regions of interest (ROIs) and then normalized simplified visualization of the data. The ROIs for (A) to (C) are indicated by dashed squares in fig. S2.

Fig. 4. Dual-mode droplet actuation via ASVs.

Time-lapse images showing the droplet actuation processes (A) along the x direction under low-excitation amplitudes (i.e., 8.8 Vpp, 49.125 MHz) and (B) along the y axis using high-excitation amplitudes (i.e., 14.8 Vpp, 49.125 MHz). (C) The time-elapsed trajectory of the nanoliter droplets actuated by the ASVs. The color scale indicates the horizontal velocity of the droplet. (D) Internal streaming inside a trapped droplet (observed from the bottom) is visualized by stacking the time-elapsed trajectory of 10-μm polystyrene particles. (E) Particles are concentrated within the droplet after 60 s of trapping. (F) Image of a contactless fluid processor with an array of 64 independent dmIDT units. (G) The time-elapsed motion of the droplet along the transducer array. The dmIDT units are denoted as U^mn (m, n = 1, 2, 3 …). The purple shading indicates a transducer unit (U⁴⁴) that initiates the change in the direction of droplet movement. As the droplet is being held by unit U⁴⁴, the excitation signal is switched from x mode to y mode. (H) Simultaneous trapping of eight droplets operating in y mode. Scale bars, 500 μm (A to E) and 1 mm (G and H). The purple shadings indicate excited IDTs. Photo credits: Peiran Zhang, Duke University.

Fig. 5. Contactless, unidirectional droplet gating and routing via ASVs.

(A) Contactless, unidirectional droplet gating. The “f” indicates the working frequency of the corresponding transducer, from left to right: 45.125, 52.125, and 49.125 MHz. The initial position of the droplet is marked by the white dashed circle. (B) The time-elapsed unidirectional circular routing of a droplet using 24 interconnected dmIDTs triggered by periodic frequency-modulated signals. The device background is removed to show the droplets more clearly. The black arrows and lines indicate the directions of droplet motion. Scale bars, 1 mm. Photo credits: Peiran Zhang, Duke University.

Fig. 6. Active gating and bifurcated routing of droplets via ASVs.

(A) Schematics and images of an active droplet gating device. The red arrows indicate the direction of droplet movement once the gating vortex is formed. The white arrow indicates the default droplet movement direction. (B to E) Time-lapse droplet trajectories when the gating signal (i.e., f₄, 41.59 MHz) is (B) OFF and (C to E) ON. (B) The droplet cannot pass the gate and remains in the input channel when the gate is OFF. (C and D) The droplet passes the gate when the gating signal is ON. (E) When the frequency-shifting signal is reversed, the droplet can move from the output channel to the input channel. (F) Schematics and device image of a bifurcated droplet routing unit via ASVs. The red shaded area indicates the location of the ASVs that form the virtual path for droplet transportation. Output port P₁, Port₁ (left turn); P₂, Port₂ (right turn). (G and H) Automatic bifurcated droplet routing using different frequency-modulated signals, S_IL [left turn (G)] and S_IR [right turn (H)]. Scale bars, 500 μm. Photo credits: Peiran Zhang, Duke University.