This work discusses the flow of granular materials (e.g. sand). Even though a single particle is a simple object, the collective behavior of billions of particles can be very complex. In a... Show moreThis work discusses the flow of granular materials (e.g. sand). Even though a single particle is a simple object, the collective behavior of billions of particles can be very complex. In a surprisingly large amount of cases, it is not exactly known how a granular material behaves, and this while these kinds of materials are omnipresent in everyday life, industry, and nature Similar to materials such as water, which can occur as ice, liquid water and vapor, sand can also exist in different phases of matter. If you for instance walk on the beach, sand behaves like a solid, but if you pour it out of your shoes afterwards, it flows like a liquid. This thesis is dedicated to experiments where we investigate what happens when you try to "liquefy" sand by weakly vibrating it. We use an experimental setup which enables us to study how much stress is required to make sand flow, depending on the desired flow rate and the amount of vibrations. These experiments reveal several physical principles that turn out to be important towards understanding the unique behavior of these kinds of materials Show less
When soft, repulsive particles, like foam bubbles or emulsion droplets, are sheared, they show interesting scaling behaviour. We develop a simple scaling model that captures the rheological... Show moreWhen soft, repulsive particles, like foam bubbles or emulsion droplets, are sheared, they show interesting scaling behaviour. We develop a simple scaling model that captures the rheological behavior starting from three assumptions that explicitly depend on the microscopic interactions. This model starts from three ingredients: energy conservation, the concept of an effective steady state strain in our flowing system and a constitutive elasticity equation linking the effective strain to the shear stress. Our model allows for non-linear microscopic particle interactions and it predicts that the global rheological behaviour depends on the details of the microscopic interactions between the particles - in contrast to standard critical scaling theory. We test our model in computer simulations of soft, massless particles under steady shear and find that the numerics are broadly consistent with our model. jamming, rheology, foam, critical scaling Show less
This thesis presents two lines of research. On the one hand, we investigate heterogeneity in supercooled glycerol by means of rheometry, small-angle neutron scattering, and fluorescence imaging. We... Show moreThis thesis presents two lines of research. On the one hand, we investigate heterogeneity in supercooled glycerol by means of rheometry, small-angle neutron scattering, and fluorescence imaging. We find from the rheological experiments that supercooled glycerol can behave like weak solids at temperatures well above the glass transition. This is very surprising because glycerol is supposed to be purely liquid-like in this temperature range. However, the structural origin of this solid-like state of glycerol still remains unclear. The preliminary results from small-angle neutron scattering show that the solidified glycerol is structurally different from both the supercooled liquid and the crystal. In addition, fluorescence imaging of a thin film of glycerol doped with fluorescent probes reveals heterogeneous patterns of the fluorescence intensity, which is related to long-lived and micrometer scale density fluctuations in supercooled glycerol. All these results will contribute to understanding heterogeneity or even glass transition of supercooled liquids. On the other hand, we study the conformational dynamics of polyprolines by single-molecule FRET (F_rster resonance energy transfer) combined with temperature-cycle microscopy, a novel technique developed in our group, and demonstrate the potential of this new method to address complex molecular dynamics, for example the dynamics of protein-folding, at the single-molecule level. Show less
We study the shear flow of two-dimensional foams, i.e., a monolayer of bubbles floating on a soapy solution. We successfully connect local and global flow behaviour
