Univ Rennes, CNRS, IPR (Institut de Physique de Rennes), UMR 6251, F-35000, Rennes, France.
Soft Matter. 2023 Mar 15;19(11):2090-2098. doi: 10.1039/d2sm01618d.
Aqueous foams are unstable and age by drainage and coarsening. Today, these effects are well described, as also their impact on foam properties. In that respect, the foam viscoelastic properties evolve in time as a consequence of coarsening which tends to increase the mean bubble size. Here, we investigate the reverse coupling, and study if and how the continuous flow of a foam can impact its dynamics of coarsening. We introduce a new protocol where brief oscillatory measurements are inserted during a constant steady shear, allowing us to monitor the relative variation of the bubble size with time (obtained from the one of the elastic modulus ') as a function of the applied shear rate. It turns out that the coarsening rate is strongly impacted by the applied shear: this rate is continuously reduced above a critical shear rate, which itself decreases with the bubble size. This coarsening-rate reduction is interpreted as the result of out-of-equilibrium and shear-dependent film thicknesses, being higher than at rest. The critical shear rate, above which films are dynamically sustained at higher thickness than at equilibrium, emerges from the competition between the rate of rearrangements and the time required to drain the thick film created during the rearrangement. We thus report here a first experimental proof and measurements of out-of-equilibrium film thicknesses within a sheared foam, and of the impact this has on coarsening.
水基泡沫是不稳定的，会通过排水和变粗而老化。如今，这些效应已经得到了很好的描述，以及它们对泡沫性能的影响。在这方面，泡沫的粘弹性性质随着变粗而随时间演变，这往往会增加平均气泡尺寸。在这里，我们研究了相反的耦合，并研究了泡沫的连续流动是否以及如何影响其变粗动力学。我们引入了一种新的方案，即在恒定的稳态剪切过程中插入短暂的振荡测量，使我们能够监测气泡尺寸随时间的相对变化（从弹性模量的变化中获得）作为施加剪切速率的函数。结果表明，变粗速率受到施加剪切的强烈影响：在临界剪切速率以上，该速率连续降低，而临界剪切速率本身随气泡尺寸的减小而减小。这种变粗速率的降低被解释为非平衡和剪切依赖的膜厚度的结果，其高于静止时的厚度。高于该临界剪切速率，膜在更高的厚度下在动态下得以维持，而不是在平衡状态下，这是由重排速率和在重排过程中形成的厚膜排干所需的时间之间的竞争所决定的。因此，我们在这里首次报道了在剪切泡沫中测量非平衡膜厚度及其对变粗的影响的实验证据。
