The “Shadow Effect” in Colloid Transport and Deposition Dynamics in Granular Porous Media: Measurements and Mechanisms

The role of hydrodynamic and colloidal interactions in the transport and deposition dynamics of colloidal particles in granular porous media is systematically investigated. Colloid transport experiments were conducted with three suspensions of positively charged colloidal latex particles (133, 288, and 899 nm in diameter) and negatively charged packed quartz grains. The column experiments were carried out under a wide range of solution ionic strengths (10-5.5−10-2.0 M) and approach velocities (10-4.5−10-2.5 m/s) until a complete breakthrough was attained, thus allowing unambiguous determination of the maximum attainable surface coverage for each deposition run. Results show that the rate of blocking and the maximum attainable surface coverage are determined by a unique interplay between flow intensity, particle size, and solution ionic strength. It is suggested that the tangential or shear component of the fluid flow around collector grains creates a "shadow zone" on the collector surface down gradient of deposited particles where the probability of subsequent deposition is substantially reduced. The shadow zone is determined by the combined effect of hydrodynamic interaction and electrostatic double layer repulsion. Increasing the approach velocity and particle size and decreasing the solution ionic strength result in a larger area of the shadow zone and hence reduced maximum attainable surface coverages. It is also proposed that sand grain surface roughness influences the dynamics of particle deposition by creating shadow zones down gradient of surface protrusions where particle deposition is significantly hindered.