Abstract: 
We use and develop simple experimental methods for studying the behavior of soft materials. I will discuss two recent examples, both focussed on the mechanical response of soft hydrogel materials.

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1. Surprising re–swelling of microgels after an osmotic compression

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I will discuss osmotic shock experiments on soft, permeable objects, such as microgel particles, porous beads used in chromatography, or biological cells. We use dedicated microfluidic devices to capture such soft objects and subject them to a rapid change in the applied osmotic pressure. We observe a surprising non-monotonic compression / re-swelling behavior: Following the osmotic shock, the particle volume initially decreases rapidly, as expected. However, at longer time scales we observe a slow reswelling, during which the particle goes back approximately to its initial volume.

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I will present a simple physical explanation of the effect, along with a simple model that accounts for the data. Our approach enables, based on one single experiment, direct experimental access to three key physical properties of porous soft objects: their elastic bulk modulus, their permeability to an aqueous background liquid, and the mobility of the osmotic agent, the macromolecules used to apply the osmotic pressure, within the pores of the soft object.

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2. Mechanics from Calorimetry: A new probe of elasticity for responsive hydrogels 

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We illustrate and exploit the inherent thermodynamic link between thermal and mechanical properties in temperature-responsive hydrogels by showing that the compressive elastic modulus of PNIPAM hydrogels can be probed using differential scanning calorimetry. Interestingly, this enables us to measure mechanical properties without any knowledge of forces or stresses exerted on the material. We validate our approach by using conventional osmotic compression tests. The method could be particularly valuable for determining the mechanical response of thermosensitive submicron-sized and/or oddly shaped particles, to which standard methods are not readily applicable.

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References:
[1] “Compression and reswelling of microgel particles after an osmotic shock” J.F. Sleeboom, P. Voudouris, M.T.J.J.M. Punter, F.J. Aangenendt, D. Florea, P. van der Schoot, H.M. Wyss Physical Review Letters, 119, 098001 (2017)

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[2] “Mechanics from Calorimetry: Probing the Elasticity of Responsive Hydrogels” F.J. Aangenendt, J. Mattsson, W.G. Ellenbroek, H.M. Wyss Physical Review Applied 8, 014003 (2017)