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 de position dynamics of colloidal particles in granular porous media is system atically investigated. Colloid transport experiments were conducted with th ree suspensions of positively charged colloidal latex particles (133, 288, and 899 nm in diameter) and negatively charged packed quartz grains. The co lumn experiments were carried out under a wide range of solution ionic stre ngths (10(-5.5)-10(-2.0) M) and approach velocities (10(-4.5)-10(-2.5) mis) until a complete breakthrough was attained, thus allowing unambiguous dete rmination of the maximum attainable surface coverage for each deposition ru n. Results show that the rate of blocking and the maximum attainable surfac e coverage are determined by a unique interplay between flow intensity, par ticle size, and solution ionic strength. It is suggested that the tangentia l or shear component of the fluid flow around collector grains creates a "s hadow zone" on the collector surface down gradient of deposited particles w here the probability of subsequent deposition is substantially reduced. The shadow zone is determined by the combined effect of hydrodynamic interacti on and electrostatic double layer repulsion. Increasing the approach veloci ty and particle size and decreasing the solution ionic strength result in a larger area of the shadow zone and hence reduced maximum attainable surfac e coverages. It is also proposed that sand grain surface roughness influenc es the dynamics of particle deposition by creating shadow zones down gradie nt of surface protrusions where particle deposition is significantly hinder ed.