Efficient Propulsion and Hovering of Bubble-Driven Hollow Micromotors underneath an Air-Liquid Interface.

Bubble-driven micromotors have attracted substantial interest due to their remarkable self-motile and cargo-delivering abilities in biomedical or environmental applications. Here, we developed a hollow micromotor that experiences fast self-propulsion underneath an air-liquid interface by periodic bubble growth and collapse. The collapsing of a single microbubble induces a ∼1 m·s-1 impulsive jetting flow that instantaneously pushes the micromotor forward. Unlike previously reported micromotors propelled by the recoiling of bubbles, cavitation-induced jetting further utilizes the energy stored in the bubble to propel the micromotor and thus enhances the energy conversion efficiency by 3 orders of magnitude. Four different modes of propulsion are, for the first time, identified by quantifying the dependence of propulsion strength on microbubble size. Meanwhile, the vertical component of the jetting flow counteracts the buoyancy of the micromotor-bubble dimer and facilitates counterintuitive hovering underneath the air-liquid interface. This work not only enriches the understanding of the propulsion mechanism of bubble-driven micromotors but also gives insight into the physical aspects of cavitation bubble dynamics near the air-liquid interface on the microscale.

Pillar array microtraps with negative dielectrophoresis.

We present a microfluidic particle-trap array that utilizes negative dielectrophoresis (nDEP) force and hydrodynamic force. The traps are located at the stagnation points of cylindrical pillars arranged in a regular array, and they can function as both single-particle traps (capable of discriminating particles based on size) and multiparticle traps (capable of controlling the number of particles trapped). By adjusting the relative strength of the nDEP and hydrodynamic forces, we are able to control the number of trapped particles accurately. We have used 5 microm polystyrene beads to validate and demonstrate the capability of this new particle-trap design. Pulsed nDEP was used to increase the selectivity and stability. Good correlation between simulation and the experimental results was obtained.

Separation of particles by pulsed dielectrophoresis.

In this paper, we introduce a dielectrophoresis (DEP)-based separation method that allows for tunable multiplex separation of particles. In traditional DEP separations where the field is applied continuously, size-based separation of particles uses the cubic dependence of the DEP force on particle radius, causing large particles to be retained while small particles are released. Here we show that by pulsing the DEP force in time, we are able to reverse the order of separation (eluting the large particles while retaining the small ones), and even extract mid-size particles from a heterogeneous population in one step. The operation is reminiscent of prior dielectrophoretic ratchets which made use of DEP and Brownian motion, but we have applied the asymmetric forces in time rather than in a spatial arrangement of electrodes, thus simplifying the system. We present an analytical model to study the dynamic behavior of particles under pulsed DEP and to understand the different modes of separation. Results from the model and the experimental observations are shown to be in agreement.