FILM STRAINING OF COLLOIDS IN UNSATURATED POROUS-MEDIA - CONCEPTUAL-MODEL AND EXPERIMENTAL TESTING

A film-straining theory is introduced, which proposes that transport o f suspended colloids can be retarded due to physical restrictions impo sed by thin water films in partially saturated porous media. A quantit ative, mechanistic model is provided to predict the film-straining eff iciency. In this model, the concepts of ''critical matric potential'' and ''critical saturation'' are introduced, at which thick film interc onnections between pendular rings are broken and film straining begins to become effective. The modeled magnitude of colloid transport throu gh water films depends on the ratio of colloid size to film thickness and on flow velocity. Effective penetration of hydrophilic colloids th rough unsaturated porous media is predicted when a system is above the critical saturation value. For colloids smaller th an the thickness o f adsorbed thin water films, the model predicts that colloids can stil l be efficiently transported, even when the system matric potential an d saturation are lower than their critical values. The model was teste d through experiments on transport of hydrophilic latex particles (fou r sizes from 0.01 to 1.0 mu m) in sand columns of three different grai n sizes and at flow rates spanning 4 orders of magnitude. The conceptu al basis of this model is supported by,good agreement between the wide range of experiments and model predictions using only two adjustable parameters.