Wed August 26th 2015
16:00 – 17:00
Seminar Studying Complex Fluids with Simple Experiments
Hans Wyss


We use and develop experimental tools to study the structure, dynamics and rheology of soft materials, thereby revealing the physical mechanisms that govern their behavior. To do so, we often use very simple experimental setups based on microfluidic devices. I will discuss the following two examples of recent research:
1. Long–range repulsion of colloids driven by ion–exchange and diffusiophoresis
I will first present recent results from our experimental study of so-called exclusion zone formation, an effect where colloidal particles are expelled from an interface over distances of up to hundreds of micrometers. This effect has been observed in various disciplines and near various materials, including cells and biological tissues, metals, and ion-exchange resins. However, the physical mechanism responsible has remained unclear. Using targeted experiments that minimize the effects of gravity and convection, we show that this exclusion zone formation is caused by a combination of ion-exchange at the interface, and diffusiophoresis of particles in the resulting ion gradients [1]. Our results enable a precise prediction and control of the behavior, with possible applications in water purification, separation technologies, or novel anti-fouling surfaces.
2. Colloidal glasses and gels made from soft particles
I will further discuss the behavior of materials consisting of soft particles, where knowledge on the viscoelastic properties of the single particles is of key importance for elucidating the macroscopic viscoelastic response. To characterize the properties of a single particle we have developed new tools, including Capillary Micromechanics [2,3], and microfluidic particle traps that enable applying a rapid osmotic shock to the particles. I will also briefly discuss the consequences of particle softness on the colloidal glass transition, and show that by using soft particles the concept of the dynamical fragility, known from molecular glass formers, can be directly extended to colloidal glasses [4].
1. D. Florea, S. Musa, J. M. Huyghe, H. M. Wyss PNAS 111 (18) 6554-6559 (2014).
2. H. M. Wyss, T. Franke et al. Soft Matter 6 (18), 4550–4555 (2010)
3. T. Kong, L. Wang, H. M. Wyss, and H. C. Shum Soft Matter 10 (18) 3271-3276 (2014)
4. J. Mattsson, H. M. Wyss, A. Fernandez-Nieves, et al. Nature 462 (7269), 83 (2009).
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The 10th Complex Motion in Fluids 2021
Max Planck Gesellschaft
Centre for Scientific Computing