Design and Fabrication of a Membrane-Based Sensor for Capillary Pressure Measurement in 2D Micromodels

Abstract:

Capillary pressure and capillarity are central to the description of multiphase flow in porous media. For decades, practical and theoretical descriptions of multiphase flow in porous media have been inevitably relying on empirical relations between capillary pressure and phase saturation, which have long been recognized to be hysteretic. Extensive studies have been devoted to understanding and mitigating such hysteresis in hope of achieving a unique description of the state of the porous medium flow system. In such an effort, direct in-situ measurement of pore-scale capillary pressure would be extremely valuable to identify new physics as well as to validate new hypotheses and theoretical and numerical models. However, due to a number of experimental challenges, on-chip measurement of pore-scale capillary pressure is still lacking. Although some designs have been proposed for pressure measurement in microfluidic devices, few if any of them are suitable for measurement in multiphase flow in porous media. To that end, we aim to design and fabricate an on-chip sensor that enables direct capillary pressure quantification within individual pores in 2D porous micromodels. The micromodel used in the current study is fabricated in polydimethylsiloxane (PDMS) using soft lithography with a thin membrane incorporated which deflects subject to pressure variations in the fluid flow. With this technique, a 2D pressure field can be inferred by means of a pre-calibrated correlation between the membrane deflection in the z-direction and pressure change. A microscope coupled with a high-speed camera is employed to provide optical readout, allowing for possible simultaneous quantification of other flow characteristics, such as velocity fields, phase distribution and interfacial area. By this experiment, we hope to provide a novel method for direct quantification of capillary pressure at the pore scale and this study will lead to a renewed understanding of pore-scale physics of multiphase flow in porous media.