该研究团队进行了在微重力环境下进行的湿泡沫粗化实验，其中液相分数达到其去拥挤点及以上，以避免重力排水。随着时间的推移，实验达到一种自相似状态，其中归一化气泡尺寸分布是不变的。让人惊讶的是，分布的特点是小漫游气泡过量，移动网络内堵塞较大的气泡。这些漫游的气泡让人想起粒状材料(不受接触力影响的颗粒)中的嘎声。研究人员确定了一个临界液相分数φ *，在该值之上气泡组装去拥挤，同时两个气泡组分合并成一个狭窄的气泡液态分布。
意外的是，φ *大于泡沫的随机紧密堆积分数φrcp; 这是因为，在φrcp和φ*之间，由于气泡之间的粘附性较弱，大气泡保持连接。该研究提出的模型确定了解释研究人员观察结果的物理机制。他们对湿泡沫中的粗化现象提出了一种新的综合观点。本研究结果可应用于其他相分离体系，也可用于控制具有分层结构的固体泡沫的细化。
据悉，两相体系的粗化对于致密颗粒填料（例如合金、泡沫、乳液或过饱和溶液）的稳定性至关重要。平均场理论预测具有宽粒度分布的渐近标度状态。水性泡沫是研究粗化诱导的结构良好模型体系，因为连续的液相和分散的气相是均匀的和各向同性的。
附：英文原文
Title: Hierarchical bubble size distributions in coarsening wet liquid foams
Author: Galvani, Nicolò, Pasquet, Marina, Mukherjee, Arnab, Requier, Alice, Cohen-Addad, Sylvie, Pitois, Olivier, Hhler, Reinhard, Rio, Emmanuelle, Salonen, Anniina, Durian, Douglas J., Langevin, Dominique
Issue&Volume: 2023-9-14
Abstract: 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 φ* , above which the bubble assembly unjams and the two bubble populations merge into a single narrow distribution of bubbly liquids. Unexpectedly, φ* is larger than the random close packing fraction of the foam φrcp. This is because, between φrcp and φ* , 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.
DOI: 10.1073/pnas.2306551120
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2306551120