  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.
