Flow inside the Rayleigh-Plateau Instability. By reducing the interfacial tension between inner drops and the middle phase, the dynamics of fluid flow inside the Rayleigh-Plateau instability is visualized. Poiseuille flow is clearly visible in the left hand side of the picture and videos shown below. In the region between pinch-off and the Poiseuille flow, the fluid slows down as seen in the absence of curvature in the inner drops. As the outer drop pinches off, and the flow substantially slows down to a near halt, the inner drops are no longer affected by the flow and thus change shape to spherical drops and start to coil inside the drop that pinches off from the jet. The inner drops have charged surfactants complicating the flow dynamics.
Droplet Imaging Velocimetry (DIV): Visualizing Fluid Flow with Ultra-Low Surface Tension Droplets
The usual methodology of visualizing fluid flow is with tracer particles using particle imaging velocimetry (PIV); however, tracer particles are not ideal in droplet based microfluidic experiments since they have to be very small, typically nanometers, to avoid clogging devices or to avoid altering the flow dynamics in micron size channels. Thus, they are difficult to see even with an optical microscope. Furthermore, fluorescent tracer particles can be used but this requires a fluorescent microscope to detect their fluorescence. Even still, the signal contains a lot of noise, and thus achieving an accurate flow profile is still challenging. In this new technique, named droplet imaging velocimetry (DIV), water droplets with ultra-low surface tension are used to directly map the velocity profile of fluid flow in an immiscible fluid. Here we show the flow profile during jetting as double emulsions are generated in a microfluidic device as seen in the upper left image. This technique provides valuable information about fluid properties of drop formation.