Ultralight graphene-based cellular elastomers are found to exhibit nearly frequency-independent piezoresistive behaviors. Surpassing the mechanoreceptors in the human skin, these graphene elastomers can provide an instantaneous and high-fidelity electrical response to dynamic pressures ranging from quasi-static up to 2000 Hz, and are capable of detecting ultralow pressures as small as 0.082 Pa.

Microscopic water-in-oil droplets are a versatile chemical and biological platform whose dimensions result in short reaction times and require minuscule amounts of reagent. Methods exist for the production of droplets, though the vast majority are only able to do so in continuous flows, restricting the ability to independently control reactions of individual droplets, a prerequisite for programmable digital microfluidics. Here we present a novel method to produce individual picoliter-scale droplets on-demand using surface acoustic waves (SAW). Acoustic forces arising from SAW act on the oil-water interface, creating a droplet whose volume is defined by the applied power, duration of the force and system geometry. Additionally, this method is able to pre-concentrate particles simultaneously with droplet production, meaning that particles and cells, even if in a dilute mixture, can be easily encapsulated. Our method is expected to be applicable to high-throughput screening, bioreactor creation and other microfluidic processes.

Individual droplets can be isolated within microfluidic systems by use of an immiscible carrier layer. This type of two phase systems, often termed "digital microfluidics", find wide ranging applications in chemical synthesis and analysis. To conduct on-chip biochemical analysis, a key step is to be able to merge droplets selectively in order to initiate the required reactions. In this paper, a novel microfluidic chip integrating interdigital transducers is designed to merge multiple droplets on-demand. The approach uses surface acoustic wave induced acoustic radiation forces to immobilize droplets as they pass from a channel into a small expansion chamber, there they can be held until successive droplets arrive. Hence, no requirement is placed on the initial spacing between droplets. When the merged volume reaches a critical size, drag forces exerted by the flowing oil phase act to overcome the retaining acoustic radiation forces, causing the merged volume to exit the chamber. This will occur after a predetermined number of droplets have merged depending on the initial droplet size and selected actuation power.

We present a method for sensitive and tunable particle sorting that we term virtual deterministic lateral displacement (vDLD). The vDLD system is composed of a set of interdigital transducers (IDTs) within a microfluidic chamber that produce a force field at an angle to the flow direction. Particles above a critical diameter, a function of the force induced by viscous drag and the force field, are displaced laterally along the minimum force potential lines, while smaller particles continue in the direction of the fluid flow without substantial perturbations. We demonstrate the effective separation of particles in a continuous-flow system with size sensitivity comparable or better than other previously reported microfluidic separation techniques. Separation of 5.0 μm from 6.6 μm, 6.6 μm from 7.0 μm and 300 nm from 500 nm particles are all achieved using the same device architecture. With the high sensitivity and flexibility vDLD affords we expect to find application in a wide variety of microfluidic platforms.

Abstract-Limit cycle, or self-oscillations, can occur in a variety of NEMS devices illuminated within an interference field. As the device moves within the field, the quantity of light absorbed and hence the resulting thermal stresses changes, resulting in a feedback loop that can lead to limit cycle oscillations. Examples of devices that exhibit such behavior are discussed as are experimental results demonstrating the onset of limit cycle oscillations as continuous wave (CW) laser power is increased. A model describing the motion and heating of the devices is derived and analyzed. Conditions for the onset of limit cycle oscillations are computed as are conditions for these oscillations to be either hysteretic or nonhysteretic. An example simulation of a particular device is discussed and compared with experimental results.[1190]Index Terms-Finite element method (FEM), laser drive, limit cycle oscillation, self-oscillation, thermal stress.