Galvani Nicolò, Pasquet Marina, Mukherjee Arnab, Requier Alice, Cohen-Addad Sylvie, Pitois Olivier, Höhler Reinhard, Rio Emmanuelle, Salonen Anniina, Durian Douglas J, Langevin Dominique
Sorbonne Université, CNRS, Institut des NanoSciences de Paris, Paris 75005, France.
Lab Navier, Univ Gustave Eiffel, Ecole Nationale des Ponts et Chaussées, CNRS, Champs-sur-Marne 77420, France.
Proc Natl Acad Sci U S A. 2023 Sep 19;120(38):e2306551120. doi: 10.1073/pnas.2306551120. Epub 2023 Sep 14.
Coarsening of two-phase systems is crucial for the stability of dense particle packings such as alloys, foams, emulsions, or supersaturated solutions. Mean field theories predict an asymptotic scaling state with a broad particle size distribution. Aqueous foams are good model systems for investigations of coarsening-induced structures, because the continuous liquid as well as the dispersed gas phases are uniform and isotropic. We present coarsening experiments on wet foams, with liquid fractions up to their unjamming point and beyond, that are performed under microgravity to avoid gravitational drainage. As time elapses, a self-similar regime is reached where the normalized bubble size distribution is invariant. Unexpectedly, the distribution features an excess of small roaming bubbles, mobile within the network of jammed larger bubbles. These roaming bubbles are reminiscent of rattlers in granular materials (grains not subjected to contact forces). We identify a critical liquid fraction [Formula: see text], above which the bubble assembly unjams and the two bubble populations merge into a single narrow distribution of bubbly liquids. Unexpectedly, [Formula: see text] is larger than the random close packing fraction of the foam [Formula: see text]. This is because, between [Formula: see text] and [Formula: see text], the large bubbles remain connected due to a weak adhesion between bubbles. We present models that identify the physical mechanisms explaining our observations. We propose a new comprehensive view of the coarsening phenomenon in wet foams. Our results should be applicable to other phase-separating systems and they may also help to control the elaboration of solid foams with hierarchical structures.
两相体系的粗化对于诸如合金、泡沫、乳液或过饱和溶液等致密颗粒堆积的稳定性至关重要。平均场理论预测了一种具有广泛粒径分布的渐近标度状态。水性泡沫是研究粗化诱导结构的良好模型体系，因为连续液相以及分散气相都是均匀且各向同性的。我们在微重力条件下对液体分数高达其堵塞点及以上的湿泡沫进行粗化实验，以避免重力排水。随着时间的推移，会达到一种自相似状态，此时归一化的气泡尺寸分布保持不变。出乎意料的是，该分布的特征是存在过量的小游动气泡，它们在被堵塞的较大气泡网络中移动。这些游动气泡让人联想到颗粒材料中的 rattlers（未受接触力作用的颗粒）。我们确定了一个临界液体分数[公式：见原文]，高于此分数时气泡集合体会解除堵塞，并且两种气泡群体合并为单一的窄分布气泡液体。出乎意料的是，[公式：见原文]大于泡沫的随机密堆积分数[公式：见原文]。这是因为，在[公式：见原文]和[公式：见原文]之间，由于气泡之间的弱粘附力，大气泡仍保持连接。我们提出了一些模型，这些模型确定了解释我们观察结果的物理机制。我们提出了一种关于湿泡沫粗化现象的全新综合观点。我们的结果应适用于其他相分离体系，并且它们也可能有助于控制具有分级结构的固体泡沫的制备。
