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Abstract: Fingering is a hydrodynamic instability that occurs when a more mobile fluid displaces a fluid of lower mobility. When the primary source of the mobility difference is viscosity, the instability is termed viscous fingering. Viscous fingering is often, though not always, undesirable in industrial processes, particularly secondary petroleum recovery. Linear stability analysis by Hejazi et al. has indicated that the production of a non-monotonic viscosity profile can stabilize the interface. Herein, we use step-growth polymerization at the interface between two miscible monomers as a model system. In particular, a dithiol monomer displaced a diacrylate that reacted to form a linear polymer that behaves as a Newtonian fluid. Viscous fingering was imaged in a horizontal Hele-Shaw cell via Schlieren, which is sensitive to changes in index of refraction, and therefore polymer conversion. By varying reaction rate via initiator concentration along with flow rate via a syringe pump, we were able to demonstrate increasing stabilization of the flow with increasing Damköhler number. Graphical abstract: [Figure not available: see fulltext.].

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.