(161e) Criteria for Drop Generation in Microfluidic Devices

Drop generation in microfluidic devices is an active area of research because of its relevance to a variety of applications, including high-throughput disease screening and combinatorial experimental investigations. Many applications rely on the controlled generation of drops; others use co-flowing streams of immiscible fluids. Controlling the transition between drop generation and co-flowing fluids is crucial to the performance of these devices. We present a quantitative criterion for this transition. The criterion is derived from an analysis of the critical state for unidirectional two-phase flow. Drop generation is not possible beyond a critical ratio of the volumetric flow rates of the dispersed-phase and continuous-phase fluids. Our theory shows that the critical state depends on the ratio of the fluid viscosities, the cross-section geometry of the microchannel, and the contact angle formed between the dispersed-phase fluid and the channel walls. The magnitude of surface tension does not affect the transition. Theoretical predictions are compared to experimental measurements. The theory helps explain the hysteretic behavior that is sometimes observed.