Microfluidic elongation of viscous droplets at vanishing interfacial tension

The microflow behavior of liquid-liquid dispersions is experimentally investigated in the presence of miscible fluid additives. Original microfluidic methods are developed to characterize the dynamic response of mobile droplets to a local change of interfacial tension with the external phase using sequential injections and stratified flows. Various oil-alcohol combinations are examined to unravel out-of-equilibrium fluid interactions at short timescales and clarify the role of flow rates and fluid properties on individual droplet deformations. Functional relationships are developed to characterize the initial droplet strain as well as rates of elongation during immersion and submersion stages in hydrodynamic focusing junctions. It is shown, in particular, that the droplet deformation process is highly nonlinear with the initial droplet size and that droplet growth coefficients remain constant for fluid additives having ultralow interfacial tension. Overall, this study shows the possibility to manipulate the morphology of strongly elongated droplets to enhance oil interfacial area in miscible organic solvents using microchannels.