Department of Chemical Engineering, Columbia University, New York, New York 10027, USA.
Soft Matter. 2017 Jun 21;13(24):4370-4380. doi: 10.1039/c7sm00723j.
Key features of the mechanical response of amorphous particulate materials, such as foams, emulsions, and granular media, to applied stress are determined by the frequency and size of particle rearrangements that occur as the system transitions from one mechanically stable state to another. This work describes coordinated experimental and computational studies of bubble rafts, which are quasi-two dimensional systems of bubbles confined to the air-water interface. We focus on small mechanically stable clusters of four, five, six, and seven bubbles with two different sizes with diameter ratio σ/σ ≃ 1.4. Focusing on small bubble clusters, which can be viewed as subsystems of a larger system, allows us to investigate the full ensemble of clusters that form, measure the respective frequencies with which the clusters occur, and determine the form of the bubble-bubble interactions. We emphasize several important results. First, for clusters with N > 5 bubbles, we find using discrete element simulations that short-range attractive interactions between bubbles give rise to a larger ensemble of distinct mechanically stable clusters compared to that generated by long-range attractive interactions. The additional clusters in systems with short-range attractions possess larger gaps between pairs of neighboring bubbles on the periphery of the clusters. The ensemble of bubble clusters observed in experiments is similar to the ensemble of clusters with long-range attractive interactions. We also compare the frequency with which each cluster occurs in simulations and experiments. We find that the cluster frequencies are extremely sensitive to the protocol used to generate them and only weakly correlated to the energy of the clusters.
无定形颗粒材料（如泡沫、乳液和颗粒介质）对所施加应力的机械响应的主要特征取决于颗粒重排的频率和大小，这些重排发生在系统从一种机械稳定状态过渡到另一种状态的过程中。这项工作描述了对气泡筏的协调实验和计算研究，气泡筏是限制在气液界面的准二维气泡系统。我们专注于具有两种不同尺寸的四个、五个、六个和七个气泡的小机械稳定簇，其直径比 σ/σ ≃ 1.4。专注于小气泡簇，它们可以被视为更大系统的子系统，使我们能够研究形成的簇的全部集合，测量簇出现的各自频率，并确定气泡-气泡相互作用的形式。我们强调几个重要结果。首先，对于 N > 5 个气泡的簇，我们使用离散元模拟发现，与长程吸引力相互作用相比，短程吸引力相互作用会导致更大数量的不同机械稳定簇。在具有短程吸引力的系统中，额外的簇具有更大的簇周边相邻气泡之间的间隙。实验中观察到的气泡簇集合与具有长程吸引力相互作用的簇集合相似。我们还比较了模拟和实验中每个簇出现的频率。我们发现，簇频率对生成它们的方案非常敏感，与簇的能量仅弱相关。
