Dynamic analysis of laser-induced bubbles in liquid PDMS during microFLIB technique and its possible mechanism

Laser-induced bubbles can be formed by focusing a laser beam into a liquid. Exploiting this phenomenon, we have developed a microfabrication technique for polydimethylsiloxane (PDMS) using nanosecond laser-induced bubbles, referred to as microFLIB (microfabrication using laser-induced bubbles). This technique enables rapid microfluidic fabrication on and inside a PDMS substrate because the hollow bubbles maintain their shape in uncured liquid PDMS. However, the mechanism for bubble retention during the microFLIB process remains unclear. Therefore, in the present study, we analyzed the PDMS membrane structures formed during laser-induced bubble generation; in previous studies, this membrane was assumed to retain the bubble shape. We investigated the presence of the membrane structures and found that their volumes can be controlled by the laser irradiation conditions or by the viscosity of the liquid PDMS. In addition, time-resolved pump-probe imaging was conducted to clarify the bubble behavior. Bubbles generated in the liquid PDMS maintained their shape for ∼2.5 s, whereas bubbles in silicone oil with the same viscosity as the liquid PDMS gradually shrank and collapsed. The results provide insights into the possible mechanism for bubble retention associated with the PDMS membrane structures that surround the bubbles formed during the microFLIB process.