INTERNAL FLOW IN MICRO-DROPLETS
Micro-droplets generated in micro-fluidic devices act as micro-reactors for different applications. Due to advantages like requirement of reduced sample volumes, small device size, flexibility in operations and high surface area to volume giving rise to increased heat and mass transfer, it is widely used for biological and chemical applications. In micro-fluidic devices, viscous forces and the interfacial tension are dominant than inertial forces and gravitational forces, yielding a low Reynold’s number with laminar flow. Being in laminar flow regime, achieving mixing between the fluids are inherently difficult. However, internal recirculation patterns are observed in micro-droplets generated in microfluidic devices. These flows are generated by the drag force on the contact surfaces with the surrounding walls of the channel and the liquid-liquid interface. These flow patterns vary with the depth of the droplet and with the shape of droplet. A smaller droplet experiences internal flow due to the drag force exerted by the continuous fluid and a larger droplet (comparable to channel dimensions) also experiences additional shear force exerted by the walls of the channel. Presence of surfactants (normally added to stabilise the droplets) also leads to internal flow due to Marangoni effect. Enhancement of these internal flow inside the droplet, increases the rate of reaction which can be beneficial for several applications. So, understanding the dynamics of flow and the several factors affecting these flow patterns namely, surface tension gradient, flow rates of the phases, viscosity and surface roughness of the channels and even external fields like electrical and magnetic field are important. A high speed particle tracking and analysis of the flow patterns are used in the study to understand the mixing in droplets. More insights into the droplets mixing behaviour can be used to enhance them for enhancing the reactions in the droplets.
Keywords: Droplet, microfluidics, Reynold’s number, Circulation, Marangoni flow