Visco-plastic sculpting

Visco-plastic lubrication (VPL) has been established as a method for reliably suppressing interfacial instabilities and enhancing flow stability for multi-layer systems. Here we extend this methodology to the formation of shaped interfaces in multifluid core-annular configurations. We study multi-layer VPL flows in which we perform both experiment and computation with oscillating the flow rates of the individual phases. According to the flow rate variations we succeed in freezing in a range of different interfacial patterns. Experiments performed with carbopol as lubricating fluid, and with xanthan and polyethylene oxide solutions as core fluid, serve to illustrate the potential of the method. We show that single pulsed changes in the imposed inflow rates can result in small interface indentations that remain frozen into the interface as it propagates downstream. Repeated pulses produce periodically patterned interfaces. We are able to control the frequency and amplitude of the interfacial patterns, but not directly the shape. Inelastic core fluids have been observed to produce rounded bulges whereas elastic core fluids have produced diamond shapes. Moreover, numerical simulations extend the range of shapes achievable and give us interesting insights into the forming process. (C) 2014 AIP Publishing LLC.