Abstract
Viscous fingering is a widely observed phenomenon, in which finger-like instabilities occur at the interface of two fluids, whenever a less viscous phase displaces a more viscous phase. This instability is notoriously difficult to control, especially for given viscosity ratio and geometry. Here we demonstrate experimentally the active control of viscous fingering of two given liquids, for given geometry and flow rate in a Hele-Shaw cell. The control is realized by taking advantage of electro-osmotic flows along the surfaces confining the fluid, via applying an external electric field. Depending on the direction of electric field, the induced secondary electro-osmotic flows either assist or oppose the hydraulic flow, effectively reducing or increasing the flow resistance, leading to the control of interface stability. The mechanism of apparent “electrokinetic thinning/thickening” is proposed to explain the experimental observations. Theoretical predictions of linear stability are confirmed experimentally for a broad range of immiscible electrolyte displacements.
Highlights
Viscous fingering is a widely observed phenomenon, in which finger-like instabilities occur at the interface of two fluids, whenever a less viscous phase displaces a more viscous phase
Saffman and Taylor first conducted experiments in a “Hele-Shaw cell”, a flow apparatus consisting of two parallel glass plates separated by a thin gap, and performed linear stability analysis to show that the onset of instability is controlled by a single parameter, the viscosity ratio
We demonstrate that active control of viscous fingering in a Hele-Shaw cell is possible by applying external electric fields parallel to the flow direction
Summary
Viscous fingering is a widely observed phenomenon, in which finger-like instabilities occur at the interface of two fluids, whenever a less viscous phase displaces a more viscous phase. Finger-like patterns form when a lower viscosity fluid (e.g. water) displaces a more viscous one (e.g. oil), effectively creating paths of lower hydraulic resistance[18]. This problem was originally studied by Hill[19] in the early 1950s, and soon followed by Saffman and Taylor[20] and Chuoke et al.[21]. Saffman and Taylor first conducted experiments in a “Hele-Shaw cell”, a flow apparatus consisting of two parallel glass plates separated by a thin gap, and performed linear stability analysis to show that the onset of instability is controlled by a single parameter, the viscosity ratio. The extent of this active control depends on the magnitude of the applied electric field
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