Pore fouling by microparticles: experimental and numerical investigations
G.-C. Agbangla1, 2, P. Bacchin1, 2 and E. Climent2, 3
1
Université de Toulouse; INPT, UPS Laboratoire de Génie Chimique;
118 Route de Narbonne - 31062 Toulouse Cedex 4, France
(E-mail : agbangla@chimie.ups-tlse.fr; bacchin@chimie.ups-tlse.fr)
2
CNRS; Fédération de recherche FERMAT; Toulouse, France
3
Université de Toulouse; INPT, UPS, Institut de Mécanique des Fluides;
Allée Camille Soula, F-31400 Toulouse, France
(E-mail : climent@imft.fr)
ABSTRACT
Pore fouling by microparticles is still unpredictable and stays an important problem to optimize membrane and filtration
processes. A scientific challenge is to know how the parameters of these processes affect the capture of particles and the
clogging of the filter. In this work, we developed a poly-dimethylsiloxane (PDMS) device which allows a direct and
dynamic observation of the clogging of microchannels (10 and 20 micrometers wide) by micrometrics particles (2.5, 3.5
and 5 micrometers in diameter). The experiments put in light the formation of different 3D clogging patterns according
to the filtration conditions (particle/wall surface interaction, direction of the flowing conditions, ratio particle/pore size).
These results are analysed through 3D dynamic simulation of the transport of particles in the flowing suspension
accounting for hydrodynamic and surface interactions. The analysis of these experimental and numerical results helps
identifying deposition, interception, bridging mechanisms in pore fouling phenomena.
Keywords. Membrane – Clogging – Micro particle – Particle interaction – Aggregation
INTRODUCTION
In separation processes such as microfiltration or ultrafiltration, one of the most important problems is the fouling of the
porous material which reduces the efficiency of processes (Belfort et al. 1994). To limit or to avoid the impact of the
fouling of filter or membrane, it is necessary to know how these phenomena occurs and how they are influenced by the
parameters of the process (i.e. the filtered dispersion, the filter, and the filtering conditions). Different experimental
techniques and numerical approaches have been used in order to progress on this point but fouling is still unpredictable.
A possible cause for these discrepancies between simulation and experiments is linked to the existence, when filtering
small particles (less than 10 micrometers), of physico-chemical surfaces interactions (linked to the colloidal properties
of the matter) which affect the way particles interacts with the filter wall and with others particles. Fouling phenomena
result of a complex interplay between hydrodynamics and these surface interactions forces occuring at nanometric scale
(Bacchin et al. 1995). In this context, we investigate the dynamic capture and aggregation of particles at a pore entrance
with a two way coupling between simulation and experimental approaches.
MATERIAL AND METHODS
On an experimental point of view, a key to breakthroughs in the understanding of fouling phenomena is the
development of novel, non-invasive, in situ quantification of physico-chemical processes occurring during filtration
(Chen and Elimelech 2004). To answer to this need, we developed poly-dimethylsiloxane (PDMS) microfluidic devices
to mimic filtration systems. Our device consists in a parallel arrangement of several microchannels being 10 or 20 m
wide (to mimic a set of pore in a filter). The direction of the flowing suspension can be parrallel to the filtration area (to
mimic cross flow filtration) or perpendicular to the area (to mimic dead-end filtration). These devices allow a direct
observation (with video or confocal microscopy) of the fouling and the clogging of microchannels by microparticles.
The microparticles are monomodal latex microspheres (2.5, 3.5 and 5 m in diameter) negatively charged. The
dispersion is injected in the device at a volume fraction between 0.001 and 0.05 with a constant flow rate (2mL/h)
leading to a laminar flow (Re=0,45) in the channels. The magnitude of the electrostatic interaction between particles
and the PDMS wall is changed by modifying the salt concentration between 10 -5 M and 10-1 M (these concentration
staying below the concentration leading to particle aggregates).
On a numerical point of view, we develop a simulation code using the Force Coupling Method (Climent & Maxey,
2010) to describe the transport of particle in the 3D laminar pore flow. The basic idea of the Force Coupling Method
relies on the multipole decomposition of forcing terms accounting for the velocity perturbation induced by the presence
of the particles in the flow. These forcing terms are spread over spherical Gaussian envelopes and added to the NavierStokes equations. This numerical approach permits to solve simultaneously the coupled equations of the particles
motion and the dynamic response of the flow. Our numerical study is focused on a simple geometry consisting in a
sudden reduction of section similar to the microchannel entrance used in the experiments. The interplay between fluid
stresses and DLVO interactions (attraction/repulsion forces) is accounted for by using this method to simulate the fluid
flow and trajectories of colloidal particles in the microchannel and then to examine the capture and aggregation of
particles in such a geometry.
RESULTS
The experimental device allows the direct observation of particle capture and aggregation by video microscopy in the
filtration area (at the entrance of microchannels). The salt concentration has an important effect on the capture rate. No
particle capture is observed when the particles are dispersed in pure water whereas particle capture is clearly seen when
particles are dispersed in 10-1 M. These results show that the suspension stability had an important effect on particle
capture: when the salt concentration increased, the repulsive electrostatic interaction between the particles was reduced,
which promoted particle-particle adhesion. For particles dispersed in 10 -1 M in KCl, different 3D fouling patterns can be
observed when the effect of different parameters is screened. As an example, for some specific physico-chemical
conditions (spherical latex suspensions of 5 µm in diameter diluted in 10-1 M KCl salt solution), it leads to the
formation of dendrites on the channel wall. The length could reach more than 100 µm, i.e. 20 particle diameters after
120 minutes. As seen in figures, the formation of these dendrites is influenced with the flowing direction: straight and
tilted dendrites are obtained respectively in dead end filtration (Fig. 1) and cross flow filtration (Fig. 2). We will show
that the tilt of dendrites is mostly due to the change in streamlines induced by the flow. The numerical code allows
describing the dynamic balance between hydrodynamic and surface interactions which controls the formation of
different aggregate structures and leads to the filter clogging. The two way coupling between the numerical and
experimental results allows identifying the mechanisms leading to pore fouling
Figure 1: Clogging of microchannels by microparticles in
dead-end flow. Image taken after two hours of filtration
Figure 2: Clogging of microchannels by microparticles
in cross flow. Image taken after two hours of filtration
CONCLUSION
This work shows that pore fouling is controlled by a balance between physico-chemical surface interactions and
hydrodynamics forces. A slight change in this balance leads to different particle capture and pore fouling patterns.
These effects are put in light and analyzed with experimental and numerical approaches then allowing to progress in the
understanding of pore fouling.
REFERENCES
Belfort G, Davis RH, Zydney AL 1994, «The behavior of suspensions and macromolecular solutions in cross-flow microfiltration», J. of mem
science, 96, 1-58
Bacchin Patrice, Aimar Pierre, Sanchez Victor 1995, «Model for colloidal fouling of membrane», AIChE Journal, 41 368-376
Chen JC, Li Q, Elimelech M, 2004, «In situ monitoring techniques for concentration polarization and fouling phenomena in membrane filtration»,
Adv. Colloid Interface Sci, 107; 83: 108.
E. Climent & M.R. Maxey 2010, «The Force Coupling Method: A flexible approach for the simulation of particulate flows», inserted in “Methods for
creeping flows”, Ressign Press, Eds F. Feuillebois and A. Sellier.