Flow characteristics in microfluidic devices is naturally laminar due to the small channel dimensions. Mixing based on molecular diffusion is generally poor. In this article, we report the fabrication and characterization of active surface-acousticwave-driven micromixers which exploit the acoustic streaming effect to significantly improve the mixing efficiency. A side-by-side flow of water and fluorescent dye solution was driven by a syringe pump. Surface wave with a frequency of 13 MHz was launched perpendicular to the flow. The wave was generated by two designs of interdigitated electrodes on LiNbO 3 substrate: parallel electrodes and focusing electrodes. The mixing efficiency was observed to be proportional to the square of the applied voltage. Under the same applied voltage, the focusing type offers a better mixing efficiency. The fabrication of the micromixer is compatible to current technology such as soft lithography and deep reactive ion etching. Despite the high throughput and fast mixing time, the mixer design is simple and could be integrated into any microfluidic platform.

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This paper reports the fabrication and characterization of an adhesive-based liquid-metal microcoil for magnetic resonance relaxometry (MRR). Conventionally, microcoils are fabricated by various techniques such as electroplating, microcontact printing and focused ion beam milling. These techniques require considerable fabrication efforts and incur high cost. In this paper, we demonstrate a novel technique to fabricate three-dimensional multilayer liquid-metal microcoils together with the microfluidic network by lamination of dry adhesive sheets. One of the unique features of the adhesive-based technique is that the detachable sample chamber can be disposed after each experiment and the microcoil can be reused without cross-contamination multiple times. The integrated microcoil has a low direct-current (DC) resistance of 0.3 Ω and a relatively high inductance of 67.5 nH leading to a high quality factor of approximately 30 at 21.65 MHz. The microcoil was characterized for ∼0.5 T proton MRR measurements. The optimal pulse duration, amplitude, and frequency for the 90° pulse were 131 μs, -30 dB (1.56 W) and 21.6553 MHz, respectively. In addition, we used the liquid-metal microcoil to perform a parametric study on the transverse relaxation rate of human red blood cells at different hematocrit levels. The transverse relaxation rate increases quadratically with the hematocrit level. The results from the liquid-metal microcoil were verified by measurements with a conventional solenoid coil.

Droplet coalescence plays an important role in droplet-based microfluidics. This letter reports the phenomenon of thermocoalescence of two droplets in a chamber with an microheater. An integrated resistive sensor allows the measurement of heating temperature. The merging process was investigated at different flow rates. Experimental results showed that the droplet slows down at increasing temperature and eventually merges with the subsequent droplet. Coalescence occurs at a critical heating temperature. The letter discusses the relationship between droplet velocity, merging temperature and flow rates.

Manipulation of a micromotor's locomotion has been the ultimate aim of scientists and engineers alike. While numerous roadmaps have been cast, the interswitching of the locomotion and directionality of these miniaturized machines remains elusive. In this report, ultrasound is utilized to produce stop/go motion on bubble‐propelled micromotors via Bjerknes forces. An intricate study using high‐speed camera on the interactions between the bubbles and micromotor is undertaken. The reciprocal action between oscillating bubbles aggregates and ejected microbubbles in an acoustic field demonstrate influence on the motion of the micromotor. Long‐range translational motion can be induced into the micromotor, when repulsive forces between bubble aggregates and ejected microbubbles are manifested in an acoustic field by Bjerknes forces. Additionally, such ultrasonic pulses demonstrate capability to change the directionality of the micromotor, where chirality of the locomotion can be switched. Here, introduction of pulses of ultrasonic irradiation demonstrates new capabilities to switch the motion of bubble‐propelled micromotors.