Colloid acceleration and
dispersion in saturated
micromodels
Donald Bren School of Environmental Science & Management
University of California, Santa Barbara
Maria Auset & Arturo Keller
December 8th 2003
1
Effective Pore Diameter
Clay
Silt
Sand
Gravel
Contaminants
Protozoa
Bacteria
Viruses
Macromolecules
Molecules
Colloids
Size Range (µm)
2
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.
3
Issue
0.50
0.45
KCl
0.40
MS2
0.35
0.30
C/C0
Studies at the macro scale have
observed different behavior
of colloids compared to
conservative tracer,
explained as size exclusion.
0.25
0.20
0.15
0.10
0.05
0.00
0.7
Sirivithayapakorn and Keller. Water Resources research. 2003.
0.8
0.9
1.0
1.1
1.2
1.3
Pore Volume
Objective
Investigate transport (velocity and dispersion) of
different sized colloids in different geometries at the
pore scale using micromodels.
4
1.4
Channel
width
CAD design
PDMS channels
Micromodel
10 µm
Narrow
20 µm
Wide
10 and 20
µm
Zigzag
1000 microns
100 microns
1000 microns
5
Experimental Setup
VCR/monitor
Inlet
Video image
Video
camera
-Residence times,
-Particle trajectories,
-Dispersion coefficients,
For different
Micromodel
pressure gradients.
Microscope
Outlet to flow
PC with video
capture board
Injection of monodisperse suspension of colloids
20
microns
7 µm
1000 microns
2 µm
6
7
Residence time VS colloid diameter
11
Residence time (s)
10
9
Wide 100 Pa
8
7
Wide 1500 Pa
6
Narrow 100 Pa
Minimum
pressure
5
4
Narrow 1500 Pa
Zigzag 100 Pa
3
Zigzag1500 Pa
2
Maximum
pressure
1
0
1
2
3
4
5
6
7
Colloid diameter (um)
8
9
8
“Acceleration” VS Inlet velocity
3
"Acceleration factor"
Narrow micromodel
2.5
7um Narrow
2
2um Narrow
W ide micromodel
7um Wide
1.5
2um Wide
1
7um ZigZag
2um ZigZag
Zigzag micromodel
0.5
0
0
0.05
0.1
0.15
Inlet velocity, cms
Acceleration factor =
-1
Colloid velocity
Water velocity
9
7 µm
Regular wide micromodel
Flow direction
10
2 µm
Regular wide micromodel
Flow direction
11
7 µm and 2 µm
Zig zag micromodel
Flow direction
12
Dispersion coefficient VS velocity
DL 
 L2
2t


2
1 N di  L
DL  
N i
2t i
Dispersion coefficient, 10-6 cm2s-1
80
7 um
70
ZIGZAG
5 um
3 um
60
2 um
50
40
30
WIDE
20
10
NARROW
0
0
1
2
3
4
5
Velocity, 10-2 cms-1
6
7
8
13
Dispersivity VS colloid size
14
Dispersivity = -0.4712x + 13.753
R2 = 0.9437

DL

Dispersivity, 10-4 cm
12
ZigZag
10
Wide
8
Narrow
6
Dispersivity= -0.3881x + 5.1746
R2 = 0.7804
4
Dispersivity = -0.2217x + 1.718
2
R2 = 0.9712
0
0
1
2
3
4
5
6
7
8
Colloid size, um
14
Discussion
Hydrodynamic
r
r
chromatography
r
Exclusion from
detouring streamlines
15
Conclusion
• As colloid size increases and/or pore width decreases:
- Particles move more rapidly than a conservative tracer.
- Dispersion decreases.
- Dispersivity decreases.
• 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.
16
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.
17