Particle aggregation and flow patterns induced by ultrasonic standing wave and acoustic streaming: An experimental study by PIV and PTV

Abstract

In this study, two flow phenomena induced by the acoustic force, ultrasonic standing wave (USW) and acoustic streaming, were examined by means of flow visualization (FV), particle image velocimetry (PIV) and micro particle tracking velocimetry (μPTV). The control of the aggregation of particles in a suspension solution by ultrasonic standing wave (USW) and acoustic streaming was studied for a PMMA (Poly(methyl methacrylate)) cuvette of length 12.6 mm, width 12.6 mm and height 46 mm. The fluid inside was filled with water and μm-size particles as the suspension solution, and the flow is excited with a piezoelectric plate operating at MHz frequency oscillation of voltage signal. This oscillation generates USW inside the liquid space, while at the same time induce acoustic streaming flows in the confined space. Three particle sizes (5–18 μm) were tested with frequencies vary from 0.847 to 2.032 MHz in the present study. Experimental results show that the ultrasonic standing wave is successfully generated, and the error between the standing wavelength in the theory and in the experiment does not exceed 6%. USW is accompanied by the acoustic streaming flow, and within the performed frequency range, the degree of aggregation for particles in different diameters is 18 µm > 10 µm > 5 µm. With all driving frequencies it is found that the acoustic streaming flow starts as the oscillation is turned on and affecting the flow patterns immediately. At driving frequencies of 0.847, 1.863, 2.032 MHz for 10 µm and 18 µm particles, a stable streaming flow coexists with the USW pattern that cause the flow to move with minimum reduction of the particle aggregation can be observed.The micro-scale flow patterns of acoustic streaming flows induced by a single triangular obstruction were investigated by flow visualization (FV) and μPTV. Different base to altitude ratios from 0.25 to 4 of the isosceles triangular obstructions were tested, corresponding to the top angles varied from 127° to 18°, the heights from 0.3 mm to 0.6 mm, and the base widths from 1.2 mm to 0.1 mm. By using the piezoelectric plates, the steady acoustic streaming flow patterns were successfully created and observed at a range of driving voltages from 5 to 35 V and frequencies from 0.5 to 3 kHz. The vector field calculated using μPTV results show that the streaming flow is more concentrated at the vertex of the triangular obstructions, and flow acceleration at the vertex is more significant in the case of small base to altitude ratio. These experimental results suggest the probabilities to utilize both acoustic streaming and USW for applications such as particle sorting and manipulation.