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Periodic Report Summary 2 - SOFTEVA (Drying of complex fluids on soft substrates)

Publishable summary:
The main objective of SOFTEVA project is to describe the deposition of colloidal suspensions on soft
materials on two main aspects:
* the coating uniformity
* the measurement of stresses induced by drying.
Two types of soft materials are in the scope of this project: elastomers for which no solvent exchange
occurs with the aqueous colloidal suspension and hydrogels making posssible solvent exchange.
The project has also the ambition to study two different types of colloidal suspensions: concentrated
silica nanoparticles that enables the development of large stresses and continous films and micron
size particles used in a very dilute regime.
The work performed since the beginning of the project consists in the following.
1. experimental and theoretical investigation of the drying of silica nanoparticles on thin elastomeric
2. experimental and theoretical investigation of the absorption of a water drop containing
microspheres on a swelling hydrogel.

Regarding item 1. two papers have been published on the measurement of the drying stress of silica nanoparticles. The principle of the first paper is to deposit a milimeter size drop on a thin milimetric disk of elastomer. The first technique consisted in measuring the bending of a thin elastic sheet, which can be related theoretically, with a mechanical model, to the drying stress.
These experiments highlighted the feasibility of such measurements, the order of magnitudes of the drying stress and a first analysis of the particle size effect.
A second study has been designed to improve the measurement sensitivity. To do so, we deposit a drop on a large floating elastic sheet. The tension induced by the drying triggers a wrinkling instability. The quality of these experiments led to discover the effect of the particle size distribution and we revealed that the largest particles dominate the macroscopic tensile stress.

Regarding item 2, we published two papers in which we investigated experimentally the absorption of a water drop containing a diluted suspension of particles on hydrogels.
First, we have evidenced that absorption, in contrast with evaporation, intrinsically leads to nearly homogeneous particle deposition on a surface. We also investigated the fine effect of the particle concentration on the contact line depinning that occurs during the absorbing process.
We have shown that the adsorbed particles increase the pinning force at the contact line and, we
developed a method to measure the receding contact angle with the consideration of the hydrogel
swelling. The key observations have been rationalized by analytical models.
In addition, we explored in a third paper an alternate method to manipulate colloids in a drop sitting on a hydrogel. We used diffusiophoresis to induce a particle motion by setting an initial salt concentration gradient between the drop and the hydrogel. We shown that either the particles can be deposited with a rapid dynamics on the gel or either repealed from the surface. Observations have been successfully rationalized by a model.

In additions to the main objectives, other scientific investigations have been developed, all related to soft matter. In these additional papers where several students have been supervised, we studied the wetting in complex geometries made by fiber assemblies. We also studied the peeling of thin and soft materials by an advancing contact line, which is a promising result toward decontamination of surfaces with a chemical free technique.

References (cited in the previous report):
- F. Boulogne, A. Sauret, B. Soh, E. Dressaire, H. Stone, "Mechanical tuning of the evaporation rate
of liquid on crossed fibers", Langmuir, vol. 31, no. 10, 2015, pp. 3094-3100.
- A. Sauret, F. Boulogne, B. Soh, E. Dressaire, H. Stone, "Wetting of fibrous arrays: liquid
morphology on two randomly oriented fibers", The European Physical Journal E, 2015
- A. Sauret, F. Boulogne, D. Cébron, E. Dressaire, H. Stone, "Wetting morphologies on an array of
fibers of different radii", Soft Matter, vol. 11, 2015, pp. 4034-4040.
- A. Sauret, F. Boulogne, J. Cappello, E. Dressaire, H. Stone, "Damping of liquid sloshing by foams",
Physics of Fluids, vol. 27, no. 2, 2015.
- J. Cappello, A. Sauret, F. Boulogne, E. Dressaire, H. Stone, "Damping of liquid sloshing by foams:
from everyday observations to liquid transport", Journal of Visualization, vol. 18, no. 2, 2015, pp.
- F. Boulogne, H. A. Stone, "Self-crumpling elastomers: bending induced by the drying stimulus of a
nanoparticle suspension", EPL, vol. 108, 2014, pp. 19001.
- F. Boulogne, F. Ingremeau, J. Dervaux, L. Limat, H. A. Stone, "Homogeneous deposition of
particles by absorption on hydrogels", EPL, vol. 112, no. 4, 2015, pp. 48004.
- Y. L. Kong, F. Boulogne, H. Kim, J. Nunes, J. Feng, H. A. Stone, "Deposition of quantum dots in a
capillary tube", Langmuir, vol. 31, no. 45, 2015, pp. 12560-12566.
- B. Andreotti, O. Baumchen, F. Boulogne, K. E. Daniels, E. R. Dufresne, H. Perrin, T. Salez, J. H.
Snoeijer, R. W. Style, "Solid Capillarity: When and How does Surface Tension Deform Soft
Solids?", Soft Matter, vol. 12, 2016, pp. 2993-2996.
- F. Boulogne, Y. L. Kong, J. K. Nunes, H. A. Stone, "Effect of the polydispersity of a colloidal drop
on the drying induced stress as measured by the buckling of a floating sheet", Physical Review
Letters, vol. 116, 2016, pp. 238001.
- F. Boulogne, F. Ingremeau, L. Limat, H. A. Stone, "Tuning the receding contact angle on hydrogels
by addition of particles", Langmuir, vol. 32, no. 22, 2016, pp. 5573-5579.
- H. Kim, F. Boulogne, E. Um, I. Jacobi, E. Button, H. A. Stone, "Controlled uniform coating from
the interplay of Marangoni flows and surface-adsorbed macromolecules", Physical Review Letters,
vol. 116, 2016, pp. 124501.

References (published since the previous report):
- F. Boulogne, F. Ingremeau and H. A. Stone, "Coffee-stain growth dynamics on dry and wet surfaces", Journal of Physics: Condensed Matter, vol. 29, no. 7, 2017, pp. 074001.
- F. Boulogne, S. Khodaparast, C. Poulard, H. A. and Stone, "Protocol to perform pressurized blister tests on thin elastic films", The European Physical Journal E, vol. 40, no. 6, 2017, pp. 64.
- F. Boulogne, S. Shin, J. Dervaux, L. Limat, H. A. Stone, "Diffusiophoretic manipulation of particles in a drop deposited on a hydrogel", Soft Matter, vol. 13, 2017, pp. 5122-5129.
- S. Khodaparast, F. Boulogne, C. Poulard, H. A. and Stone, "Water-based peeling of thin hydrophobic films", Physical Review Letters, vol. 119, Oct 2017, pp. 154502.
- A. Sauret, F. Boulogne, K. Somszor, E. Dressaire, H. A. Stone, "Drop morphologies on flexible fibers: influence of elastocapillary effects", Soft Matter, vol. 13, 2017, pp. 134-140.

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