Concentration of Microparticles Using Flexural Acoustic Wave in Sessile Droplets

Abstract

Acoustic manipulation of microparticles and cells has attracted growing interest in biomedical applications. In particular, the use of acoustic waves to concentrate particles plays an important role in enhancing the detection process by biosensors. Here, we demonstrated microparticle concentration within sessile droplets placed on the hydrophobic surface using the flexural wave. The design benefits from streaming flow induced by the Lamb wave propagated in the glass waveguide to manipulate particles in the droplets. Microparticles will be concentrated at the central area of the droplet adhesion plane based on the balance among the streaming drag force, gravity, and buoyancy at the operating frequency. We experimentally demonstrated the concentration of particles of various sizes and tumor cells. Using numerical simulation, we predicted the acoustic pressure and streaming flow pattern within the droplet and characterized the underlying physical mechanisms for particle motion. The design is more suitable for micron-sized particle preparation, and it can be valuable for various biological, chemical, and medical applications.

Keywords: acoustofluidic; flexural wave; numerical simulation; particle concentration.

Figure 1

(a) Schematic presentation of the acoustofluidic concentration device. (b) Principle of the concentration process. (c) Experimentally observed 10 μm particle concentration at 48 kHz, 40 Vpp. (d) The trajectory of particles in cross-sections at different heights.

Figure 2

The numerical model details. In the FE model, Γ₁ is the droplet area, Γ₂ is the PDMS area, and Γ₃ is the glass area.

Figure 3

(a) The normalized acoustic pressure distribution. (b)The normalized acoustic streaming, the arrows in the primary and enlarged area are <v₂> and <v_2x>. (c) The time-series distribution of 10 μm particles upon d_m = 10 nm. The arrow represents the velocity direction, and the color represents the magnitude of velocity, m/s.

Figure 4

(a) The intensity of acoustic streaming in the droplet of different cross-sections. The height of the droplet is 1.14 mm. (b) The force conditions for particles to gather in two positions.

Figure 5

Time-series images of 10 μm particle concentration process. (Supplemental Movie S4) The images were taken at the h = 0. The equivalent diameter of the concentrated region at 40, 60, 80, 100, 200, 300 s are 216, 228, 265, 306, 340, 356 μm respectively.

Figure 6

Particle concentration of different sizes within 300 s (48 kHz, 40 Vpp). (a–d) 7, 5, 2, and 0.5 μm particle. Where d is particle diameter. (e) 10 and 2 μm mixed particles. The images were taken at the h = 0. The equivalent diameter of the concentrated region for (a–c,e) are 352, 391, 167, and 429 respectively.

Figure 7

Simultaneously concentration of 10 μm particles and cells in multiple sessile droplets. (a) Photograph image of the chip and numbered diagram of droplets. (b) The effect of particle concentration in nine droplets at 200 s. (c) Concentration effect of liver cancer cells in three droplets.