Movement of oil droplets against salt concentration gradients in thin capillaries

• Microfluidic visualization showed movement of oil droplets in a salinity gradient. • The gradient-driven movement can drive a droplet through a narrow constriction. • Droplet motion in the capillary channel and the driving forces are discussed. • Abrupt salinity changes may be a physical mechanism of smart waterflooding. Mobilization of residual oil droplets is the key process for enhanced oil recovery. Visualization of the droplet movement at a pore level provides insights on the underlying physical mechanisms. We couple a microfluidic droplet generator and a thin glass capillary to study the movement of oil droplets under salinity gradients with visualization of individual droplet movements. The driving forces that affect the movement of the droplets are discussed. We demonstrate experimentally that oil droplets in micro-confined channels can be mobilized and move against pressure under the concentration gradients of dissolved salts. The gradient-driven movement can be strong enough to drive a droplet through a narrow constriction in the middle of the capillary channel. The droplet movement can be understood by combining a Marangoni stress due to surfactant redistribution, electrostatic interaction and diffusiophoresis. This suggests that the abrupt change of salinity may be one of the physical mechanisms of smart waterflooding.