Water-oil drainage dynamics in oil-wet random microfluidic porous media   analogs
  Displacement experiments carried out in microfluidic porous media analogs show that reduced surface tension leads to a more stable displacement, opposite to the process in Hele-Shaw cells where surface tension stabilizes the displacement of a more viscous fluid by a less viscous fluid. In addition, geometry of porous media is observed to play an important role. Three random microfluidic porous media analogs were made to study water-oil drainage dynamics, featuring a pattern of randomly connected channels with a uniform width, a pattern with Gaussian channel width distribution, and a pattern with large isolated pores. The microfluidic chips fabricated using Polydimenthylsiloxane with glass covers have the internal surface treated by Trichlorosilane to achieve a uniform oil-wet condition. The aqueous phase displaces the oil phase, with a viscosity ratio of about 1:40 and a density ratio of 1:0.85. Videos 1-3 show water flooding processes. It is observed that both channel size distribution (Video 2) and heterogeneity in pore size (Video 3) lead to stronger fingers and reduced displacement efficiency. Video 4 shows that meniscus in small channels retreat as water front moves into a nearby large cavity due to the disparity in the capillary force and contact angle hysteresis. Videos 5 and 6, both taken at 100X magnification in Chip 2, show the stabilizing effect of reduced interfacial tension. 
Wei Xu, Jeong Tae Ok, Keith Neeves, Xiaolong Yin
physics.flu-dyn