1 Colloid acceleration and dispersion in saturated micromodels Donald Bren School of Environmental Science & Management University of California, Santa Barbara Maria Auset & Arturo Keller December 8 th 2003

Size Range (µm)  Colloids Molecules Macromolecules Viruses Bacteria Protozoa Gravel Sand Silt Clay Effective Pore Diameter Contaminants

3 Motivation Knowledge of colloid transport is required to efficiently manage and remediate environmental contaminants: 1.Protect drinking water aquifers. 2.Development of bioremediation strategies. 3.Microbial enhanced oil recovery.

4 Objective Investigate transport (velocity and dispersion) of different sized colloids in different geometries at the pore scale using micromodels. Studies at the macro scale have observed different behavior of colloids compared to conservative tracer, explained as size exclusion. Sirivithayapakorn and Keller. Water Resources research Issue

5 10 µm Narrow 20 µm Wide 10 and 20 µm Zigzag 1000 microns CAD design PDMS channels Micromodel 1000 microns Channel width 100 microns

6 Experimental Setup VCR/monitor Inlet Microscope Micromodel Video camera Video image PC with video capture board Outlet to flow -Residence times, -Particle trajectories, -Dispersion coefficients, For different pressure gradients. Injection of monodisperse suspension of colloids 7 µm 2 µm 1000 microns 3000 microns 20 microns

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8 Residence time VS colloid diameter

9 “Acceleration” VS Inlet velocity Acceleration factor = Colloid velocity Water velocity

10 Flow direction 7 µm Regular wide micromodel

11 Flow direction 2 µm Regular wide micromodel

12 Zig zag micromodel 7 µm and 2 µm

13 Dispersion coefficient VS velocity

14 Dispersivity VS colloid size

15 Hydrodynamic chromatography Exclusion from detouring streamlines r r r Discussion

16 As colloid size increases and/or pore width decreases: - Particles move more rapidly than a conservative tracer. - Dispersion decreases. - Dispersivity decreases. Conclusion Colloids travel faster than predicted by a tracer and traditional theory because they stay in central streamlines, which are - faster, - straighter less dispersion preferential paths Dispersion and dispersivity depend on porous media geometry and colloid size.

17 Acknowledgement Arturo Keller, (Bren School, UCSB), Sanya Sirivithayapakorn, (Bren School, UCSB), David Pine, (Chemical Engineering, UCSB), Eric Michel, (Chemical Engineering, UCSB), Ministerio Español de Educación, Cultura y Deporte.