Rheology of high-aspect ratio, Graphene platelet suspensions
S. Barwich, J.N. Coleman, M.E. Möbius, Yieldfind and flow of highly concentrated, few-layer Graphene suspensions (in review)
K.R. Paton et al., Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids, Nature materials 13, 624-630 (2014)
Bubble formation at electrodes during electrolysis
At sufficiently high overpotentials during electrolysis gas bubbles form at the electrodes. Here we study the growth and detachment of Hydrogen bubbles on Platinum electrodes.
D. Fernández, P. Maurer, M. Martine, J.M.D. Coey, M.E. Mö̈bius, Bubble formation at a gas-evolving microelectrode, Langmuir 30, 13065-74 (2014)
D. Fernández, M. Martine, A. Meagher, M. E. Möbius and J.M.D. Coey, Stabilizing effect of a magnetic field on a gas bubble produced at a microelectrode, Electrochem. Comm. 18, 28-32 (2012).
Local correlations in Random Packings
Random packings such as granular media, dense dispersions or wet foams are ubiquitous in nature. In these systems, particles do not pack into a crystalline structure, but jam into a disordered state without any long range order. Mechanical stability requires the average contact number of the particles to be at least 4 in two dimensions (or 6 in 3D) for particles without friction. However, disorder leads to a wide distribution of contact numbers in the packing. How "random" is this contact network in disordered packings? We simulate a two dimensional model system of polydisperse discs to study spatial correlations in the contact network of random packings. We find that discs with few neighbours are surrounded by discs with many neighbours and vice versa. This is analogous to the well known Aboav-Weaire law in cellular structures.
C.B. O'Donovan, M.E. Möbius, Spatial correlations in polydisperse, frictionless two-dimensional packings, Phys. Rev. E 84, 020302(R) (2011) (html,pdf).
Granular drops with J. Royer, L. Oyarte, H. Jaeger
Liquid jets are unstable and eventually form droplets due to the Rayleigh-Plateau instability that is driven by the surface tension of the liquid. Surprisingly, granular jets can give rise to a similar phenomenon despite their apparent lack of surface tension: The granular jet (in a evacuated chamber) becomes unstable and forms droplets. The droplet size is surprisingly robust and of the order of the jet diameter, analogous to the liquid jet. The experiments suggest that energy loss through inelastic collisions and tiny cohesive forces on the nano-Newton scale drive this instability. The latter provides an effective surface tension to the granular medium.
M.E. Möbius, Clustering instability in a freely falling granular jet, Physical Review E, 74, (5), 051304, (2006) (html, pdf)
J.R. Royer, L. Oyarte, M.E. Möbius, H.M. Jaeger, Rupture and clustering in granular streams, Chaos, 19, 041103 (2009) (html, pdf)
J. Royer, D.J. Evans, L. Oyarte, Q. Guo, E. Kapit, M.E. Möbius, S. R. Waitukaitis, H. M. Jaeger, High-speed tracking of rupture and clustering in freely falling granular streams, Nature 459, 1110 (2009). (html, pdf); Supplement (html, pdf).
Slushy glycerol with T. Xia, M. van Hecke, W. van Saarloos, M. Orrit
Molecular glasses are liquids that do not crystallize as the temperature is lowered. Instead, with decreasing temperature, their viscosity diverges exponentially at the glass transition, while retaining their Newtonian flow behaviour. Glycerol is one example of such a glass former. Upon slow cooling and aging near the glass transition temperature, we observe the formation of a slushy phase. Even though it appears crystalline, it retains its soft slushy consistency and does not harden any further. The study of the microstructure of this slush is still ongoing work.
M.E. Möbius, T. Xia, M. Orrit, W. van Saarloos, M. van Hecke, Aging and Solidification in supercooled glycerol, J. Physical Chemistry B 114, 7439 (2010) (html,pdf).
Shear-induced bubble diffusion in sheared foams with G. Katgert, M. van Heck
The non-linear response of foams is typically well described by the so-called Herschel-Bulkley model. The physical origin of this model remains poorly understood. In this experiment, we shear a two-dimensional foam in order to investigate the bubble rearrangement rate as a function of local shear rate. Due to the random displacement with respect to the mean flow, bubbles eventually start to diffuse. The corresponding relaxation time scale exhibits a non-linear relationship with the inverse shear rate and is shown to be related to the viscous bulk stress.
M.E Möbius, G. Katgert, M. van Hecke, Relaxation and Flow in a linearly sheared two-dimensional foam, Europhysics Letters 90, 44003, (2010) (html,pdf).
2D Foam rheology and the role of disorder with G. Katgert, M. van Hecke
The two-dimensional foam is a popular model system to study foam flow as it allows direct optical access to the bubble motion. In this experiment, we studied the flow of linearly sheared foam layers that are bounded by a glass plate on top of the foam. By taking into account the non-linear drag of the bubbles with the glass plate, we were able to infer the flow curve of the foam. Furthermore, we studied the effect of disorder. Monodisperse (=single size) bubbles that order into a hexagonal lattice give rise to a different bulk flow behaviour compared to the disordered foam.
G. Katgert, A. Latka, M.E. Möbius, M. van Hecke, Flow in linearly sheared two-dimensional foam: from bubble to bulk scale, Physical Review E. 79, 066318, (2009). (html, pdf)
G. Katgert, M.E. Möbius, M. van Hecke, Rate dependence and role of disorder in linearly sheared two-dimensional foams, Physical Review Letters 101, (5), 058301, (2008). (html, pdf)
Granular shear with X. Cheng, J. Lechman, A. Fernandez-Barbero, G. Crest, G. Karczmar, H. Jaeger, S. Nagel
When a granular medium is sheared, it typically develops shear bands - the velocity drops off exponentially and is close to zero after a few grain diameters. In this experiment we studied granular shear in a novel shear geometry - the split bottom cell - which was pioneered by Fenistein et al.. In this geometry, the velocity profiles can be made arbitrarily large. We imaged the interior granular flow non-invasively through the use of MRI (magnetic resonance imaging) and were able to study a flow transition that depends on the filling height.
X.Cheng, J.B. Lechman, A. Fernandez-Barbero, G.S. Grest, H.M. Jaeger, G.S. Karczmar, M.E. Möbius, and S.R. Nagel, Three-dimensional shear in granular flow, Physical Review Letters, 96, (3), 03800, (2006). (html, pdf)
The Brazil Nut effect with X. Cheng, G. Karczmar, H. Jaeger, S. Nagel
When a granular material with grains of different sizes and densities is shaken, the particles do not mix. Instead, the larger particles typically go to the top and it is therefore often referred to as the "Brazil Nut effect". This is a longstanding problem in granular physics and has many practical implications in industry where size separation is undesirable. Moreover, there is a density dependence - heavier particles move to the top faster then lighter ones. This counter-intuitive phenomonology can be explained by considering air pressure gradients that develop during the shaking cycle and granular convection. We studied this problem employing both magnetic resonance and high speed video imaging and developed a model that can account for the observed size and density dependence.
M.E. Möbius, X. Cheng, P. Eshuis, G.S. Karczmar, S.R. Nagel, and H.M. Jaeger, Effect of air on granular size separation in a vibrated granular bed, Physical Review E, 72, (1), 011304, (2005). (html, pdf)
M.E. Möbius, X. Cheng, G.S. Karczmar, S.R. Nagel, H.M. Jaeger, Intruders in the dust: Air-driven granular size separation, Physical Review Letters, 74, (5), 198001, (2004). (html, pdf)
M.E. Möbius, S.R. Nagel, H.M. Jaeger, Size and density separation in granular materials, Granular Material-Based Technologies, Symposium on Granular Material-Based Technologies , Boston, MA, 2002, edited by Sen, S.; Hunt, M.L.; Hurd, A.J. , 579, Materials Research Society, 115, (2003).