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Researchers from the California Institute of Technology have unveiled new insights into how acoustic forces, the forces generated by sound waves, can be used to control and trap tiny particles within fluids. Their work explores how combining sound waves with fluid movement enables the precise manipulation of nanoparticles.
In microfluidic systems, which are tiny laboratory setups that handle minute amounts of liquid, sound waves can create pressure patterns strong enough to move or trap microscopic particles. This technique, known as acoustic trapping, is already used to control biological samples like exosomes and extracellular vesicles for diagnostic and therapeutic research. However, trapping nanoparticles reliably has remained a technical challenge.
The 5 μm polystyrene seed particles are trapped in a two-dimensional 1.5 MPa pressure field in water that yields the Gor’kov potential shown in (a). Arrows in (a) indicate the total force on the seed particle, including the hydrodynamic drag from a background Poiseuille flow that reaches 30 μm s−1 at y=0 and the acoustic radiation force corresponding to the Gor’kov potential. Hydrodynamic shielding from the uniform background flow on a 1 μm target particle provided by (b) a single trapped seed particle or (c) a cluster of nine trapped seed particles. The colors and isolines depict the fraction of the background flow velocity by which the target particle is slowed down, revealing a strong shielding effect near the seed particles. At the stable trapping point of the seed particle along y=0 marked with a cross in (a), we analyze the landscape of acoustic interaction forces on a 1 μm polystyrene target particle in (d) and a modified polystyrene particle with a density of 950 kg m−3 and, consequently, a negative dipole scattering coefficient in (e). The color indicates the relative magnitude of the acoustic interaction force acting on the target particle |AIF|/max(|AIF|), while the arrows indicate the direction. In (f), we move a pair of seed and target particles along the x direction at y=0 and evaluate the ARF and AIF acting on the target particle for different radii of target particles (a=1,0.5,0.1 μm), while either the surface-to-surface distance (d) or center-to-center distance are kept constant.
The researchers discovered that effective trapping occurs when both acoustic forces and hydrodynamic interactions, the movement of particles caused by the surrounding fluid, work together. They found that the presence of larger “seed” particles helps create local flow patterns that enhance trapping efficiency for smaller nanoparticles. Without these seed particles, the acoustic effects alone are too weak to confine nanometer-scale objects.
Their numerical simulations also suggest that this combined mechanism could be tuned to selectively trap particles based on their density, opening the door to advanced methods for separating nanoscale materials. This finding has exciting implications for biotechnology, nanomedicine, and materials science, where isolating particles like extracellular vesicles or engineered nanoparticles is essential for analysis and development.
