Contact tribology also affects the slow flow behavior of granular emulsions

Recent work on suspension flows has shown that contact mechanics plays a role in suspension flow dynamics. The contact mechanics between particulate matter in dispersions should depend sensitively on the composition of the dispersed phase: evidently emulsion droplets interact differently with each other than angular sand particles. We therefore ask: what is the role of contact mechanics in dispersed media flow? We focus on slow flows, where contacts are long-lasting and hence contact mechanics effects should be most visible. To answer our question, we synthesize soft hydrogel particles with different friction coefficients. By making the particles soft, we can drive them at finite confining pressure at all driving rates. For particles with a low friction coefficient, we obtain a rheology similar to that of an emulsion, yet with an effective friction much larger than expected from their microscopic contact mechanics. Increasing the friction coefficient of the particles, we find a flow instability in the suspension. Particle level flow and fluctuations are also greatly affected by the microscopic friction coefficient of the suspended particles. The specific rheology of our "granular emulsions" provides further evidence that a better understanding of microscopic particle interactions is of broad relevance for dispersed media flows

Driven granular media : mixing, friction & activity

Sand, coffee beans and mud all belong to a class of materials that we call granular media. Despite their relevance in industry and agriculture, the flow behaviour of these materials remains poorly understood. In particular, it is unclear how specific properties of the particles, governing the interactions at the microscopic level, influence the macroscopic flow response. In practice, it is often difficult to vary particle properties, such as stiffness or friction coefficient, in a controlled way. In this thesis, we investigate flows of granular materials with well-defined particle properties, by synthesizing the particles using novel methods. In Part I of the thesis, we investigate the role of friction in shear flows of granular suspensions. We present a method to produce millimetre-sized hydrogel particles, and investigate how the chemistry of the hydrogels affects the material friction coefficient, and subsequently determine how this relates to macroscopic flow behaviour. In Part II of the thesis, we study granular materials in systems where they are not driven by the walls, but rather from within the material. We study how passive particles driven by a single magnet can aid mixing in a microfluidic mixing chip. We also take care that the pressure drop, which limits the simple use of microfluidic chips, is greatly reduced compared to commercially available solutions. Finally, we investigate the role of geometric friction in a granular material in which each particle is individually driven to rotate. The activity of these 3D-printed particles, combined with frictional coupling of rotational and translational degrees of freedom, leads to the emergence of a granular material that displays collective behaviour. The thesis is concluded with a general discussion.</p

Enlightening force chains: a review of photoelasticimetry in granular matter

A photoelastic material will reveal its internal stresses when observed through polarizing filters. This eye-catching property has enlightened our understanding of granular materials for over half a century, whether in the service of art, education, or scientific research. In this review article in honor of Robert Behringer, we highlight both his pioneering use of the method in physics research, and its reach into the public sphere through museum exhibits and outreach programs. We aim to provide clear protocols for artists, exhibit-designers, educators, and scientists to use in their own endeavors. It is our hope that this will build awareness about the ubiquitous presence of granular matter in our lives, enlighten its puzzling behavior, and promote conversations about its importance in environmental and industrial contexts. To aid in this endeavor, this paper also serves as a front door to a detailed wiki containing open, community-curated guidance on putting these methods into practice.Comment: 13 page

What is fluidity? Designing an experimental system to probe stress and velocity fluctuations in flowing suspensions

We develop a method to investigate the microscopic origin of granular fluidity. We design a Couette cell in which we can probe the flow of soft hydrogel suspensions. As we drive the suspension with a rheometer, we have access to global flow characteristics. In addition, the Couette cell has been modified to have a transparent bottom and lid, allowing for imaging of suspension characteristics in transmission, for example flow fields. We can also use transmission imaging to probe local stresses in the suspension: we use hydrogel particles composed of gelatin, which through its photoelastic properties gives access to local stress fluctuations. We thus have access to all local microscopic variables that are relevant in the understanding of granular suspensions. We show here that our setup can indeed visualize stress fields inside the suspensions and perform flow field measurements in transmission mode. We compare the profiles of the gelatin suspension to that of a polyacrylamide hydrogel suspension, to assess robustness of observed phenomenology. We find that the flow profiles for both types of hydrogels are different; gelatin suspensions feature narrower and less rate-dependent flow profiles. We speculate on the origin of the observed difference by considering the frictional properties of the suspended particles

What is fluidity? Designing an experimental system to probe stress and velocity fluctuations in flowing suspensions

We develop a method to investigate the microscopic origin of granular fluidity. We design a Couette cell in which we can probe the flow of soft hydrogel suspensions. As we drive the suspension with a rheometer, we have access to global flow characteristics. In addition, the Couette cell has been modified to have a transparent bottom and lid, allowing for imaging of suspension characteristics in transmission, for example flow fields. We can also use transmission imaging to probe local stresses in the suspension: we use hydrogel particles composed of gelatin, which through its photoelastic properties gives access to local stress fluctuations. We thus have access to all local microscopic variables that are relevant in the understanding of granular suspensions. We show here that our setup can indeed visualize stress fields inside the suspensions and perform flow field measurements in transmission mode. We compare the profiles of the gelatin suspension to that of a polyacrylamide hydrogel suspension, to assess robustness of observed phenomenology. We find that the flow profiles for both types of hydrogels are different; gelatin suspensions feature narrower and less rate-dependent flow profiles. We speculate on the origin of the observed difference by considering the frictional properties of the suspended particles

Coaxial air flow device for the production of millimeter-sized spherical hydrogel particles

We describe a method to produce millimeter-sized hydrogel particles, by dispersing aqueous droplets in an oil using a nozzle and subsequently solidifying them. We show that we can vary the size of the particles using an air flow along the nozzle. The resulting particle size can be well predicted by a simple model where a drag force generated by the air flow, adds to the weight pulling the droplet from the nozzle. Particles produced using this method have diameters ranging from 0.7 to 2.3 mm. Production rates up to 0.5 ml/min per nozzle have been achieved, which compares favorably to standard microfluidic techniques. Finally, we show that the method can be used to produce both physical and chemical gel particles and is thus highly universal

Vision for a European metrology network for energy gases

Abstract As Europe moves towards decarbonising its energy infrastructure, new measurement needs will arise that require collaborative efforts between European National Metrology Institutes and Designated Institutes to tackle. Such measurement needs include flow metering of hydrogen or hydrogen enriched natural gas in the gas grid for billing, quality assurance of hydrogen at refuelling stations and equations of state for carbon dioxide in carbon capture and storage facilities. The European metrology network for energy gases for the first time provides a platform where metrology institutes can work together to develop a harmonised strategy, prioritise new challenges, and share expertise and capabilities to support the European energy gas industry to meet stringent EU targets for climate change and emissions reductions.</jats:p