A new bubble-driven pulse pressure actuator for micromixing enhancement

We demonstrate a thermal bubble-driven loop actuator for microfluidic applications. The working principle is similar to that of human being's heart. The actuator consists of a diverging section and a circular chamber. A slim microheater was deposited on the back surface of the glass cover, facing the diverging section of the bottom substrate. The Marangoni effect switches the states of a “long” bubble during the pulse duration stage and a sphere or a circular bubble during the pulse off stage, exporting the pulse pressure and yielding a large convection heat transfer at the vapor–liquid interface. Besides, the loop configuration pumps the bulk fluid movement around the loop. The above features protect the microheater from burnout and dissipate the Joule heat generated by the pulse heating to the supporting structure of the chip substrate, keeping the chip at the room temperature level. The newly designed loop actuator can be integrated with microfluidic systems. The loop actuator has no moving parts thus it has high reliability. Because the thermal bubble never comes out of the loop actuator, the actuator is attractive for biology applications. We demonstrate the loop actuator used for mixing enhancement. The low pulse frequency such as 10 Hz yields a wavy mixing interface, but the high pulse frequencies result in a set of mixing clouds in the mixing channel. The mixing degrees are increased with increases in pulse frequencies. The fluid samples are uniformly distributed across the channel width direction for the pulse frequency of 100 Hz. The mixing degree can reach 0.94 for a short mixing length of about a couple of millimeter at the pulse frequency of 100 Hz. The fluid Reynolds number does not influence the mixing degrees in the mixing channel.