Design of mixing in microfluidic liquid slugs based on a new dimensionless number for precise reaction and mixing operations

This paper reports mixing characteristics inside a microfluidic liquid slug using the computational fluid dynamics (CFD) simulations. Each slug is modeled as a single-phase flow domain. Slug-based microfluidics offers rapid mixing by internal circulation and transport with narrow residence time distribution, making it suitable for precise reaction and mixing operations. Miniaturizing the slug size to microscale allows high interactions between the slug internal fluid and the channel wall, leading to a highly effective internal circulation. However, quantitative understanding of mixing characteristics and the influences of operating parameters on mixing rate is crucial for the design of a liquid slug that ensures desired mixing rates. The simulation results provide insights into the influences of operating parameters on slug-based mixing rates. Based on the simulation results, the modified Peclet number, ce:italic(Pe)^* = U_s d_s² / ce:italic(lD), is proposed for designing mixing in liquid slugs. A novel method using ce:italic(Pe) ^* to estimate mixing rates and design liquid slugs to obtain desired mixing rates is discussed. Using this method, both short (ms) and long (min) mixing timescales can be accessed in the same microfluidic device by simply varying the slug velocity.