The growth and collapse of a micro-bubble under pulse heating

The possibility of using a micro-thermal bubble, generated by a micro-heater under pulse heating, as an actuator for applications in micro-bio-analytical systems is investigated in this paper. The perturbation force, generated when the micro-thermal bubble is formed instantaneously, can be used to promote such actions as mixing in the solution of a micro-reactor. Under pulse heating, a specially designed non-uniform width micro-heater (10 × 3 μm²) can induce highly localized near-homogeneous nucleation and results in periodic generation of stable single bubbles in DI water. The single bubble appears precisely on the narrow part of the micro-heater with size restricted within the superheated region in the fluid. The growth and collapse of the bubble, recorded by a high-speed CCD, is shown to be asymmetric with time if the pulse width is at milliseconds in time scale. This asymmetric behavior is very much different from those in thermal ink-jet printers. The bubble behavior under different heating duration, ranging from microseconds to milliseconds, is experimentally studied. A transient 3-D heat conduction numerical simulation is carried out to study the temperature field of the fluid before the nucleation process. To evaluate the perturbation area of the micro-bubble, submicron particles with diameter of 0.96 μm were placed in the fluid and their dynamic response during the transient bubble formation is recorded.