Direct Observation of Deformation in Microgel Filtration

Abstract

Colloidal filtration processes using porous membranes suffer from productivity loss due to colloidal matter retention and continuous build-up by the retained matter. Especially during filtration of soft matter, the deformation of the individual colloids that make up the filter cake may be significant; however, this deformation and its impact remain unresolved so far. Yet, understanding the deformation on the single colloid level as well as on the ensemble level is important to be able to deconvolute filter cake properties from resistance increase of the membrane either by simultaneous internal adsorption or blocking of pores. Here, we report on the compression of a filter cake by filtrating soft microgels in a microfluidic channel in front of a model membrane. To study the single colloid deformation amorphous and crystalline domains were built up in front of the membrane and visualized on-line using confocal fluorescence microscopy while adjusting the degree of permeation, i.e., the transmembrane flux. Results show locally pronounced asymmetric deformation in amorphous domains, while the microgels in colloidal crystals approached regular polyeder shape. Increasing the flux beyond the maximum colloid deformation results in non-isochoric microgel behavior. The presented methodology enables a realistic description of complex colloidal matter deposits during filtration.

Unravelling colloid filter cake motions in membrane cleaning procedures

“…These highly ordered areas are labeled in grey indicating half-ordered and half-amorphous regions. This existence of highly ordered areas confirms microfluidic observations of partly crystalline and partly amorphous cakes for jammed colloids 41,42 . Based on experimental phase diagrams generated with pNIPAM microgels, this transition between amorphous and crystalline structures develops in the transition region from fluid to crystal behavior at effective volume factions φ eff ∼ 0.56 43 .…”

Templating the morphology of soft microgel assemblies using a nanolithographic 3D-printed membrane

“…This behavior conforms with the expected pressure distribution in the channel, were the strongest pressure gradients are in the most narrow sections. While in previous work 9 , we were able to identify the shape of deformation of individual particles in the filter cake, we can now visualize the gradient formation of filter cakes under filtration conditions on the scale of the particles. To the best of our knowledge, while such behavior could be expected, it was not visualized on the particle scale before the present work.…”

“…crystalline and amorphous regions in the filter cake [7][8][9] . We found that the crystalline regions in the filter cake facilitate the permeation of the colloids 14 .…”

Particle Movements Provoke Avalanche-like Compaction in Soft Colloid Filter Cakes

“…The colloids are in a fixed position inside the collection and start deforming and narrowing the neighboring colloids' voids. Many different physical phenomena ranging from thermal agitation to compression and deformation, 9,11 as well as interpenetration and crystallization 12,13 are significant. One phenomenon of this glass transition stage is that the packing's morphology and coordination depend on the previous filtration time and applied force.…”

“…Subsequently, minor void size changes induced by particle deformation and cake compression affect the permeation characteristic tremendously. 11,19,20 The most compressed gel layer with the smallest voids next to the membrane dominates the overall pressure loss. 5 This dense gel layer might become a stage three network without voids when filtering tiny and soft colloids at high compression or when particles break by compression.…”