Ultrafast particle and cell concentration is essential to the success of subsequent analytical procedures and the development of miniaturized biological and chemical sensors. Here, surface acoustic wave (SAW) devices were used to excite a MHz-order acoustic wave that propagates into a microlitre droplet to drive spatial concentration and separation of two different sized suspended microparticles. The rapid concentration process, occurring within just three seconds to facilitate spatial partitioning between the two particle species, exploited two acoustic phenomena acting on the suspended particles: the drag force arising from acoustic streaming and the acoustic radiation force, both driving particles in different directions. This study elucidates the very intricate and interesting interplay of physics between fluid drag and acoustic forcing on the particles within a droplet, and, for the first time, demonstrates the existence of a frequency-dependent crossover particle size that can be used to effect species partitioning: depending on the operating frequency of the SAW device and the particle size, it is possible to cause one phenomenon to dominate over the other. A theoretical analysis revealed the extent to which each force would affect the particle trajectory (particle size range: 2-31 μm), subsequently verified through experimentation. Based on these findings, 6 and 31 μm polystyrene particles were successfully partitioned in a water droplet using a 20 MHz SAW device. This study reveals the suitability of using acoustic actuation methods for the useful partitioning of particle species within a discrete fluid volume.

The ability to isolate and sort analytes within complex microfluidic volumes is essential to the success of lab-on-a-chip (LOC) devices. In this study, acoustically-excited oscillating bubbles are used to selectively trap particles, with the selectivity being a function of both particle size and density. The operating principle is based on the interplay between the strong microstreaming-induced drag force and the attractive secondary Bjerknes force. Depending upon the size of the bubble, and thus its resonant frequency, it is possible to cause one force to dominate over the other, resulting in either particle attraction or repulsion. A theoretical analysis reveals the extent of the contribution of each force for a given particle size; in close agreement with experimental findings. Density-based trapping is also demonstrated, highlighting that denser particles experience a larger secondary Bjerknes force resulting in their attraction. This study showcases the excellent applicability and versatility of using oscillating bubbles as a trapping and sorting mechanism within LOC devices.

Solid-state nanopores have been shown to be suitable for single molecule detection. While numerous modeling investigations exist for DNA within nanopores, there are few simulations of protein translocations. In this paper, we use atomistic molecular dynamics to investigate the translocation of proteins through a silicon nitride nanopore. The nanopore dimensions and profile are representative of experimental systems. We are able to calculate the change in blockade current and friction coefficient for different positions of the protein within the pore. The change in ionic current is found to be negligible until the protein is fully within the pore and the current is lowest when the protein is in the pore center. Using a simple theory that gives good quantitative agreement with the simulation results we are able to show that the variation in current with position is a function of the pore shape. In simulations that guide the protein through the nanopore we identify the effect that confinement has on the friction coefficient of the protein. This integrated view of translocation at the nanoscale provides useful insights that can be used to guide the design of future devices.

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