Granular media : flow & agitations

This thesis is about weakly driven granular flows and suspensions. Chapter 1 is an overview of the current knowledge of slow granular flows in so-called split-bottom geometries, which in essence consist of a disk rotating at the bottom of a container. In chapter 2 we study dry granular flows in this split-bottom geometry, both in the frictional, slow, rate-independent regime, and in the liquid-like, rate dependent regime which is reached for faster flows. Chapters 3-5 deal with the flow of suspensions in the same geometry. We improve the so-called index matched scanning technique, that allows 3D imaging of the suspensions. Also for the suspension we study both the slow, rate independent and the faster, rate dependent regime. In all cases we combine 2D and 3D imaging of the flow with rheological measurements. Chapter 6 is devoted to the rheology of dry, weakly vibrated granular media. In chapter 7 we revisit a classic experiment on the compaction of granular media.LEI Universiteit LeidenStichting FOMCondensed Matter Condensed Physic

Capillary Flow-MRI:Quantifying Micron-Scale Cooperativity in Complex Dispersions

Strongly confined flow of particulate fluids is encountered in applications ranging from three-dimensional (3D) printing to the spreading of foods and cosmetics into thin layers. When flowing in constrictions with gap sizes, w, within 102 times the mean size of particles or aggregates, d, structured fluids experience enhanced bulk velocities and inhomogeneous viscosities, as a result of so-called cooperative, or nonlocal, particle interactions. Correctly predicting cooperative flow for a wide range of complex fluids requires high-resolution flow imaging modalities applicable in situ to even optically opaque fluids. To this goal, we here developed a pressure-driven high-field magnetic resonance imaging (MRI) velocimetry platform, comprising a pressure controller connected to a capillary. Wall properties and diameter could be modified respectively as hydrophobic/hydrophilic, or within w ∼ 100-540 μm. By achieving a high spatial resolution of 9 μm, flow cooperativity length scales, ξ, down to 15 μm in Carbopol with d ∼ 2 μm could be quantified by means of established physical models with an accuracy of 13%. The same approach was adopted for a heterogeneous fat crystal dispersion (FCD) with d and ξ values up to an order of magnitude higher than those for Carbopol. We found that for strongly confined flow of Carbopol in the 100 μm capillary, ξ is independent of flow conditions. For the FCD, ξ increases with gap size and applied pressures over 0.25-1 bar. In both samples, nonlocal interactions span domains up to about 5-8 particles but, at the highest confinement degree explored, ∼8% for FCD, domains of only ∼2 particles contribute to cooperative flow. The developed flow-MRI platform is easily scalable to ultrahigh field MRI conditions for chemically resolved velocimetric measurements of, e.g., complex fluids with anisotropic particles undergoing alignment. Future potential applications of the platform encompass imaging extrusion under confinement during the 3D printing of complex dispersions or in in vitro vascular and perfusion studies.</p

Rheology of weakly vibrated granular media

Biological and Soft Matter Physic

Stick-slip motion in spite of a slippery contact: Do we get what we see in atomic friction?

Quantum Matter and Optic

Refractive Index Matched Scanning of Dense Granular Materials

011301Biological and Soft Matter Physic

Thin liquid films in a funnel

We explore flow of a completely wetting fluid in a funnel, with particular focus on contact line instabilities at the fluid front. While the flow in a funnel may be related to a number of other flow configurations as limiting cases, understanding its stability is complicated due to the presence of additional azimuthal curvature, as well as due to convergent flow effects imposed by the geometry. The convergent nature of the flow leads to thickening of the film, therefore influencing its stability properties. In this work, we analyse these stability properties by combining physical experiments, asymptotic modelling, self-similar type of analysis and numerical simulations. We show that an appropriate long-wave-based model, supported by the input from experiments, simulations and linear stability analysis that originates from the flow down an incline plane, provides a basic insight allowing an understanding of the development of contact line instability and emerging length scales

A simple low pressure drop suspension-based microfluidic mixer

We present a novel microfluidic mixer that is easy to fabricate and simple to use. The mixer features a single mixing chamber in which particles are driven by a moving magnet. We show that the mixing efficiency is markedly increased by addition of particles to the chip, as a result of the diffusive motion of the sheared particles. The mixer is efficient for a range of driving rates and features a low pressure drop due to its simple design, making the mixer design compatible with soft chip material. It is therefore ideal to incorporate in lab-on-a-chip devices

Advances and challenges in soft tribology with applications to foods

The last few decades have witnessed exciting progress in the understanding of soft material mechanics. Many of these advances have been inspired by, and have broad ramifications in the field of food science. One particular aim of food science is to get a better understanding of the physico-chemical mechanisms that are relevant in sensory perception and oral processing. It is recognized that not only rheological properties but also frictional properties are relevant in these processes. The frictional phenomena relevant for sensory perception can be understood by means of tribological measurements. The foods assessed are typically soft, hydrated and heterogeneous; measuring and understanding frictional properties of such materials is a challenge. Yet, also in the field of soft solid tribology, significant steps forward have been made, which now make it possible to do well controlled studies of even realistic food tribology scenarios. In this brief review, we provide a summary of recently developed experimental methods. We discuss challenges including the system dependence of a frictional measurement, and opportunities, such as mimicking in-mouth conditions by including human saliva and using tribo-pairs with similar properties to the oral surfaces. These advances lead to progress on the path towards a complete understanding of oral processing and sensory perception

Wetting dynamics of a collapsing fluid hole

The collapse dynamics of an axisymmetric fluid cavity that wets the bottom of a rotating bucket bound by vertical sidewalls are studied. Lubrication theory is applied to the governing field equations for the thin film to yield an evolution equation that captures the effect of capillary, gravitational, and centrifugal forces on this converging flow. The focus is on the quasistatic spreading regime, whereby contact-line motion is governed by a constitutive law relating the contact-angle to the contact-line speed. Surface tension forces dominate the collapse dynamics for small holes with the collapse time appearing as a power law whose exponent compares favorably to experiments in the literature. Gravity accelerates the collapse process. Volume dependence is predicted and compared with experiment. Centrifugal forces slow the collapse process and lead to complex dynamics characterized by stalled spreading behavior that separates the large and small hole asymptotic regimes

Advances and challenges in soft tribology with applications to foods

The last few decades have witnessed exciting progress in the understanding of soft material mechanics. Many of these advances have been inspired by, and have broad ramifications in the field of food science. One particular aim of food science is to get a better understanding of the physico-chemical mechanisms that are relevant in sensory perception and oral processing. It is recognized that not only rheological properties but also frictional properties are relevant in these processes. The frictional phenomena relevant for sensory perception can be understood by means of tribological measurements. The foods assessed are typically soft, hydrated and heterogeneous; measuring and understanding frictional properties of such materials is a challenge. Yet, also in the field of soft solid tribology, significant steps forward have been made, which now make it possible to do well controlled studies of even realistic food tribology scenarios. In this brief review, we provide a summary of recently developed experimental methods. We discuss challenges including the system dependence of a frictional measurement, and opportunities, such as mimicking in-mouth conditions by including human saliva and using tribo-pairs with similar properties to the oral surfaces. These advances lead to progress on the path towards a complete understanding of oral processing and sensory perception