UC Santa Barbara

Interfaces between different fluids can be unstable with regard to hydrodynamic instabilities such as viscous fingering or buoyancy-driven convection. To study such instabilities experimentally for transparent fluids, dyes or chemical indicators are most often used to track the dynamics. While the interfacial deformation can easily be tracked by color changes, it is difficult to have access to the internal flow structure for comparison with theoretical predictions. To overcome this problem, a modification of a Schlieren technique is introduced to image 3D flows during viscously driven instabilities in a horizontal Hele-Shaw cell without using any dye or chemical indicator. The method is exquisitely sensitive, readily yielding information about 3D flows in gaps under a millimeter and allowing imaging of the flow structure internal to the fingers, rather than merely imaging the flow boundary. Following a description of the technique, visualization of dynamics for nonreactive water–glycerol and reactive displacements is presented revealing previously unobserved internal flows. These flows are tentatively interpreted in terms of known theoretical predictions.