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In previous work, our group discovered a phenomenon in which a mixed polymer-salt or mixed micellar aqueous two-phase system (ATPS) separates into its two constituent phases as it flows within paper. While these ATPSs worked well in their respective studies to concentrate the target biomarker and improve the sensitivity of the lateral-flow immunoassay, different ATPSs can be advantageous for new applications based on factors such as biomarker partitioning or biochemical compatibility between ATPS and sample components. However, since the mechanism of phase separation in porous media is not completely understood, introducing other ATPSs to paper is an unpredictable process that relies on trial and error experiments. This is especially true for polymer-polymer ATPSs in which the characteristics of the two phases appear quite similar. Therefore, our group aimed to develop semiquantitative guidelines for choosing ATPSs that can phase separate in paper. In this work, we evaluated the Washburn equation and its parameters as a potential mathematical framework to describe the flow behavior of polymer-salt and micellar ATPSs in fiberglass paper. We compared bulk phase fluid characteristics and identified the viscosity difference between the phases as a key determinant of the potential for phase separation in paper. We then used this parameter to predict the phase separation capabilities of polyethylene glycol (PEG)-dextran ATPSs in paper and control the composition of the leading and lagging phases. We also, for the first time, successfully demonstrated the phase separation phenomenon in hydrogels, thereby extending its application and potential benefits to an alternative porous medium.

International audience; The Covid-19 coronavirus, SARS-CoV-2, is inactivated much faster on paper (3 h) than on plastic (7 d). By classifying materials according to virus stability on their surface, the following list is obtained (from long to short stability): polypropylene (mask), plastic, glass, stainless steel, pig skin, cardboard, banknote, cotton, wood, paper, tissue, copper. These observations and other studies suggest that SARS-CoV-2 may be inactivated by dryness on water absorbent porous materials but sheltered by long-persisting micro-droplets of water on waterproof surfaces. If such physical phenomenons were confirmed by direct evidence, the persistence of the virus on any surface could be predicted, and new porous objects could be designed to eliminate the virus faster.

In this study we investigated the underlying mechanisms of coffee stain formation on paper - a porous substrate. When a drop of aqueous liquid containing dispersed materials dries on paper, the faster water evaporation rate at the edge of the drop drives the redistribution of the dispersed materials to the edge of the drop-covered area on paper. Deegan's discovery has been accepted as the "coffee ring" phenomenon, which has wide implications. However, stains left by dried aqueous liquids on paper are not always ring-shaped. This is because, besides Deegan's coffee ring effect, the transport of dispersed materials in porous media is affected by chromatographic and filtration effects, which are highly dependent upon the properties of the substrates. In this work we clarify the influences of different mechanisms to the dispersed materials redistribution in paper. Refereed/Peer-reviewed

International audience; The aim of this work is the experimental determination of effective transport properties of porous media consisting of compacted pieces of cardboard and polyethylene (PE). The proposed method itself is more general and can be applied to many different materials and contexts. Three major transport properties were determined: porosity, tortuosity factor and permeability. Three parameters characterizing the media were varied over a wide range: the bulk density, the size of the elements entering the mix, and the proportion of cardboard and PE in the mix. The properties were measured by means of a specially designed experimental device based on miscible gas tracing. The porosity and tortuosity factor were simultaneously determined by parametric identification, based on the experimental sample output response to an inlet gas concentration step change compared to the results of a direct numerical model. Permeability was calculated in the standard way from the measurement of the pressure drop across the sample. The reproducibility of the measurements was very good. It was found that changing the material density of the medium significantly affects all three structural properties. When the bulk density is varied between 300 and 900 kg m À3 , the tortuosity factor varies in a range as large as 18-8 and the permeability decreases by a ratio of 2-3. The tortuosity factor shows unusual variation, characterized by a decrease when density is increased above 500 kg m À3. The size of the elements does not significantly affect the structural properties of the medium in the range of parameters studied.