Spontaneous rupture of polystyrene clusters trapped by acoustic tweezers: From experimental and simulation point of view

In this research, clusters of 450 μm sized polystyrene particles, suspended in a solution of sodium dodecyl sulfate (SDS), were trapped at nodes of acoustic standing waves. It was observed for the first time that under certain circumstances, the clusters of polystyrene become unstable and start spontaneous ruptures. For further investigation of the mechanism of this phenomenon, the acoustic radiation forces between two polystyrene particles in an acoustic trap were studied using the finite element method (FEM), and it was demonstrated that when the particles go toward the trap and approach each other, after a specific distance, the repulsive acoustic interparticle force dramatically builds up. Finally, when it dominates the primary radiation force, it drives the particles apart. Observations show that by increasing the concentration of SDS in the solution from 0.25 to 3 g/l, the rate of spontaneous ruptures increases, and the clusters disintegrate at smaller radii. The threshold radius of the cluster decreased from 4.29 mm (on average) at 0.5 g/l to 2.8 mm (on average) at 3 g/l. FEM methods justified this observation too. As the concentration of SDS in the fluid increases, the viscosity of the fluid rises. Moreover, simulations indicated that by increasing the viscosity of the fluid, the value of repulsive interparticle force dramatically enhances, and the clusters fall apart more quickly. At the concentration of 1 g/l, however, an unexpected instability was observed in the clusters. Following studies on the zeta potential of the particles at different concentrations showed that this unexpected insatiability of the clusters at this concentration could be attributed to electrostatic repulsion among particles. Zeta potential of the particles in different SDS solutions showed that at this concentration, the net negative charge on the particles is maximum. So, the electrostatic repulsive force intensified the repulsive interparticle force among particles and made the cluster burst at the average radius of 2.63 mm.