torstsae

• Sæter, Torstein; Pedersen, Christian; Snoeijer, Jacco H.; Salez, Thomas & Carlson, Andreas (2024). Coalescence of Elastic Blisters Filled with a Viscous Fluid. Physical Review Letters. ISSN 0031-9007. 132(7). doi: 10.1103/PhysRevLett.132.074001. Full text in Research Archive Show summary

  Pockets of viscous fluid coalescing beneath an elastic sheet are encountered in a wide range of natural phenomena and engineering processes, spanning across scales. As the pockets merge, a bridge is formed with a height increasing as the sheet relaxes. We study the spatiotemporal dynamics of such an elastohydrodynamic coalescence process by combining experiments, lubrication theory, and numerical simulations. The bridge height exhibits an exponential growth with time, which corresponds to a self-similar solution of the bending-driven thin-film equation. We address this unique self-similarity and the self-similar shape of the bridge, both of which are corroborated in numerical simulations and experiments.

• Sæter, Torstein; Galland, Olivier; Feneuil, Blandine; Haugen, Håvard Jostein & Carlson, Andreas (2023). Growth of a viscoplastic blister underneath an elastic sheet. Journal of Fluid Mechanics. ISSN 0022-1120. 964, p. 1–16. doi: 10.1017/jfm.2023.320. Full text in Research Archive Show summary

  Inspired by geological structures formed by magmatic intrusions that deform the Earth's crust, we investigate the elastohydrodynamic growth of a viscoplastic blister under an elastic sheet resting on a prewetted substrate. By combining experiments, scaling analysis and numerical simulations, we reveal new regimes for the elastoviscoplastic growth dynamics of the blister. The blister height and its apparent radius grow as h(0,t)∼t5/9 and R(t)∼t2/9 if the fluid pressure is set by bending of the sheet, and as h(0,t)∼t5/13 and R(t)∼t4/13 if the fluid pressure is set by stretching of the sheet. A plug-like flow inside the blister dictates its dynamics, whereas the blister takes a self-similar shape given by a balance of the fluid's yield stress and the pressure gradient induced by the deformation of the elastic sheet.

• Kern, Vanessa Ruth; Sæter, Torstein & Carlson, Andreas (2022). Viscoplastic sessile drop coalescence. Physical Review Fluids. 7(8). doi: 10.1103/PhysRevFluids.7.L081601. Full text in Research Archive Show summary

  The evolution of the liquid bridge formed between two coalescing sessile yield-stress drops is studied experimentally. Surprisingly, we find that the height of the bridge evolves similar to a Newtonian fluid as h0(t)∼t, before arresting at long time due the fluid's yield stress. From viscoplastic lubrication theory we find a model for the arrested interface shape based on the balance between capillary pressure and yield stress. We then solve numerically for this final arrested profile shape and find it to depend on the fluid's yield stress τy, the surface tension coefficient σ and the coalescence angle α, represented by a modified Bingham number. We also present a scaling argument for the bridge's temporal evolution using the length scale found from this arrested shape analysis and present a similarity solution for the spatial evolution of the liquid bridge.

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• Kern, Vanessa Ruth; Sæter, Torstein & Carlson, Andreas (2021). Growth and arrest of the cusp formed by coalescing yield stress drops. Full text in Research Archive Show summary

  Yield stress fluids are ubiquitous in everyday life and burgeoning in research. Here we experimentally investigate the evolution of the cusp formed by two coalescing yield stress fluid drops using high speed imaging technology. In the short-time post coalescence the formed liquid bridge grows self-similarly balanced by viscosity and surface tension. At long-time the drop's liquid-gas interface arrests in a non-equilibrium shape influenced by yield stress. The influence of yield stress, contact angle and contact angle symmetry on these initial coalescence dynamics and on the final non-equilibrium rest state of the post-coalesced drops will be discussed, and our results compared against a numerical model for the cusp evolution.

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