Modelling the Effect of Particle Size in Microfiltration

Colloidal Jamming Dynamics in Microchannel Bottlenecks

The purpose of this work is to examine the interplay between hydrodynamic conditions and physicochemical interactions from filtration experiments of microparticles. Experiments are performed in microfluidic filters with real-time visualization at pore scale. Both flow rate and pressure are measured with time to analyze the dynamics of pore clogging and permeability. Flux stepping experiments are performed at different physicochemical conditions to determine the different clogging conditions. The results allow distinguishing different clogging behaviors according to filtration conditions which are discussed by considering particle-particle and particle-wall colloidal interactions whose main characteristics are an important repulsive barrier at 0.01 mM, a significant secondary minimum at 10 mM, and low repulsive barrier at 100 mM. Clogging delay at moderate ionic strength and deposit fragility and associated sweeping out of aggregates of particles at high ionic strength are discussed from the deposit structure, specific resistance, and deposit relaxation analyses. It has also been observed that an opening angle at microchannel entrance causes rapid clogging, this effect being more pronounced when the repulsion is partially screened. Three different scenarios are discussed by analogy to crowd swarming: panic scenario (0.01 mM) where repulsion between particles induce pushing effects leading to the creation of robust arches at pore entrances; herding instinct scenario (10 mM) where the attraction (in secondary minima) between particles enhances the transport in pores and delays clogging; and sacrifice scenario (100 mM) where the capture efficiency is high but the aggregate formed at the wall is fragile.

Collective dynamics of flowing colloids during pore clogging

Based on direct numerical simulations of the coupled motion of particles and fluid, this study analyzes the collective hydrodynamic and colloidal effects of flowing microparticles during the formation of different 3D clogging patterns at a pore entrance. Simulations of flowing suspensions through a pore with various simulation conditions show that particle concentration and surface interactions play a major role in the occurrence of the bridging phenomenon (simultaneous adhesion of many particles). In the absence of DLVO repulsive forces, plugging is characterized by the temporal reduction of the bulk permeability when increasing the volume fraction of the flowing suspension up to 20%. Under these conditions, different structures of particle aggregates (from cluster to cake plug) are formed at the pore entrance yielding different evolution rates of hydrodynamic resistance to the flow. Taking into account DLVO repulsive forces in simulations for a particle concentration equal to 10%, we observe the transition from dendritic structures (for low repulsion) to dense aggregates (for high repulsion). At high DLVO repulsive forces, the scenario of pore clogging is controlled by the collective behavior of many interacting particles. It leads to the formation of a jamming phase (Wigner glass phase) with transient clusters of interacting particles at the pore entrance. The network of jammed particles collapses when the force chains among the particles are overcome by the flow stress. The build-up and the collapse of the jammed phase at the pore entrance induce temporal permeability fluctuations. According to the relative magnitude of particle-particle and particle-wall interactions, when the jammed phase is disorganized by the flow, the residual force in the network can accelerate particles and lead to particle adhesion at the wall inducing a pore blockage and a rapid reduction of the bulk permeability.

Experimental Study of Membrane Fouling during Crossflow Microfiltration of Yeast and Bacteria Suspensions: Towards an Analysis at the Microscopic Level

Microfiltration of model cell suspensions combining macroscopic and microscopic approaches was studied in order to better understand microbial membrane fouling mechanisms. The respective impact of Saccharomyces cerevisiae yeast and Escherichia coli bacteria on crossflow microfiltration performances was investigated using a multichannel ceramic 0.2 µm membrane. Pure yeast suspensions (5 µm ovoid cells) and mixtures of yeast and bacteria (1 to 2.5 µm rod shape cells) were considered in order to analyse the effect of interaction between these two microorganisms on fouling reversibility. The resistances varied significantly with the concentration and characteristics of the microorganisms. Membrane fouling with pure yeast suspension was mainly reversible. For yeast and bacteria mixed suspensions (6 g L⁻¹ yeast concentration) the increase in bacteria from 0.15 to 0.30 g L⁻¹ increased the percentage of normalized reversible resistance. At 10 g L⁻¹ yeast concentration, the addition of bacteria tends to increase the percentage of normalized irreversible resistance. For the objective of performing local analysis of fouling, an original filtration chamber allowing direct in situ observation of the cake by confocal laser scanning microscopy (CLSM) was designed, developed and validated. This device will be used in future studies to characterize cake structure at the microscopic scale.