arXiv subjects

Gregor Plohl

Publications and source records attributed to Gregor Plohl.

3 recordsLinked to original sources

arXiv2026

History-dependent discharge of compressed particle rafts

While particle-laden interfaces play a central role in many natural and industrial processes, predicting their mechanical properties remains a major challenge. These systems combine granular characteristics conferred by particle-particle contacts with elastic behavior originating from capillary interactions, making them very sensitive to their history. Using the relaxation of uniaxially compressed particle rafts through a local constriction as a model experiment, we demonstrate the existence of a reproducible and continuous aging process. Aging is observed for both front- and back-compressed rafts and is characterized by a progressive increase in particle mobility and raft deformability. Macroscopic changes are seen, for example, in the extent of relaxation and are correlated with flow modifications observed at the mesoscopic level among which are increased particle fluxes, broader shear zones and enhanced particle rearrangements. While aging can be attributed unambiguously to the constrained passage of the particles through a constriction, its microscopic origin remains hypothetical, the results suggesting that contact lines around the particles may evolve. Beyond providing new insight into the effects of raft history, the proposed constriction flow experiment offers a simple method to control and compare aging in different particulate assemblies.

cond-mat.soft↗

arXiv2024

Relaxation of particle-laden interfaces: geometric and preparation effects

The relaxation of uni-axially compressed particle rafts through a finite opening found at the opposite side is experimentally studied. Three main behaviours are identified. The lowest relaxation degree corresponds to local unjamming. The other extreme corresponds to full relaxation and is characterized by the unjamming of the entire raft. In between, intermediate relaxation is observed. The unjammed domain first grows along the compression direction with an almost constant width and possibly extends through the entire raft length. In this case, a second phase may start during which erosion enables the unjammed channel to develop normally to the compression direction. Employing different raft geometries, i.e. various length and compression levels, and openings of various widths, we rationalize the occurrence of these different behaviours, which we attribute to the mechanical robustness of the force chain network. The threshold for channel formation and erosion are interpreted as its rupture against excessive shear and elongation, respectively. By further comparing results obtained for rafts prepared according to three different mixing degrees, we evidence that these thresholds are strongly affected by the raft history and quantify these effects in terms of shift of the rupture limits.

cond-mat.soft↗

arXiv2022

Unjamming strongly compressed particle rafts

We experimentally study the unjamming dynamics of strongly compressed particle rafts confined between two fixed walls and two movable barriers. The back barrier is made of an elastic band, whose deflection indicates the local stress. The front barrier is pierced by a gate, whose opening triggers local unjamming. The rafts are compressed by moving only one of the two barriers in the vicinity of which folds form. Using high speed imaging, we follow the folded, jammed, and unjammed raft areas and measure the velocity fields inside and outside of the initially confined domain. Two very different behaviors develop. For rafts compressed by the back barrier, only partial unjamming occurs. At the end of the process, many folds remain and the back stress does not relax. The flow develops only along the compression axis and the particles passing the gate form a dense raft whose width is the gate width. For rafts compressed at the front, quasi-total unjamming is observed. No folds persist and only minimal stress remains, if any. The particles flow along the compression axis but also normally to it and form, after the gate, a rather circular and not dense assembly. We attribute this difference to the opposite orientation of the force chain network that builds up from the compressed side and branchs. For rafts compressed at the gate side, keystone particles are immediately removed which enhances local disentanglement and leads to large scale unjamming. In contrast, for back compressed rafts, the force chain network redirects the stress laterally forming arches around the gate and resulting in a limited unjamming process.

cond-mat.soft↗