Filtration is the most common method to remove particles from solution. We take it for granted that we can remove a wide range of particulate materials, such as dirt, bacteria, and viruses, from water by using filters that usually consist of porous materials, such as sand beds. Filtration is critical to obtaining safe drinking water and other liquids suitable for industrial processing. Nevertheless, instead of filtration there are technologies that use "fields", such as gravitational sedimentation, to remove particles and/or separate different kinds of particles. This project investigates how simple chemical gradients, even modest variations in salt concentration, can transport particles and achieve particle removal or separation by size without using filters or gravitational sedimentation. This approach is called diffusiophoresis, and it works for a wide range of particle sizes, is portable, can potentially be operated in field or resource-poor settings, and can be scaled to different sizes as may be required in a variety of applications. Importantly, diffusiophoresis may work for difficult separations of small particles using environmentally benign salts, when traditional filtration or sedimentation methods may be highly inefficient or even ineffective.

The objective of the research is to investigate diffusiophoretic-driven separation processes for a large class of particles, e.g. rigid particles of different size, shape and surface charge, droplets, and vesicles, in confined configurations, which typify a wide range of biological, chemical, and engineering materials and systems, e.g. porous media, structured surfaces and microdevices. Here "separation" is considered in its broadest sense, e.g. fractionation of different particles, isolation of particles from the fluid, and transport of particles to specific destinations. Diffusiophoresis refers to the migration of colloidal particles owing to osmotic pressure variations (chemiphoresis) and local electric fields generated by differences in diffusivities of cations and anions (electrophoresis). In the first aspect of this work these ideas are explored in confined geometries and separation of different particles is characterized. In particular, time-dependent diffusiophoretic strategies are described to achieve particle transport over long times. In the second aspect of this work, diffusiophoresis is created using interphase mass transfer, e.g. dissolution of CO2 gas. Consequently, approaches for removing particles in a membrane-less flow approach are described, and a new strategy for field flow fractionation is introduced. Experiments and modeling will be developed for a wide range of particles, particles of all types, rigid/soft particles, bacterial cells, vesicles. The research will achieve fundamental understanding of diffusiophoretic transport in confined spaces and improve macroscopic transport processes.

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Princeton University
United States
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