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.
Highlights
All experiments were conducted in microfluidic chips
Core features are the two side-channels that allow flushing of microgels directly in front of the model membrane for faster cake build-up, and four protruding obstacles, effectively dividing the colloidal filter cake into five parts to create five individual filter cakes
The distance of the protruding obstacles was chosen according to the view field of the microscope (Leica SP8 Confocal Laser Scanning Microscope) and the 63x lens
Summary
These results are of great importance for understanding the filtration of soft colloidal matter through membranes since the knowledge about the consequence of compressibility of colloidal filter cakes is limited at the single microgel level as well as the resulting change in hydraulic resistance and the filtration cut-off in membrane processes. Changes in the colloidal filter cake influence overall filtration performance significantly. While the elastic moduli of m-sized soft microgels are difficult to measure and prone to error[20,21,22,23], our new method using confocal image stacks of consecutive compression steps of shell-labeled microgels could enable an in situ determination of these parameters.
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