We report observations of the self-assembly of coated droplets into regular clusters at the sudden expansion of a microfluidic channel. A double emulsion consisting of a regular train of coated microdroplets was created upstream of the channel expansion, so that the inter-drop distance, droplet length, velocity and coating thickness could be varied by imposing different inlet pressures, albeit not independently. Provided that the enlarged channel remains sufficiently confined to prohibit propagation in double file, droplets can assemble sequentially into regular linear clusters at the expansion. Droplets join a cluster via the coalescence of their coating film with that of the group ahead. This coalescence occurs when the droplets approach each other to within a critical distance at the expansion, enabled by hydrodynamic interactions within the train. Clusters comprising a finite number of droplets are obtained because reconfiguration of the droplet assembly during coalescence increases the distance to the following droplet. Decreasing the inter-drop distance increases the cluster size up to a maximum value beyond which continuous clusters form. Formalising these observations in a simple model reveals that clusters of any size are possible but that they occur for increasingly narrow ranges of parameter values. Our experimental observations suggests that background experimental fluctuations limit the maximum discrete cluster size in practice. This method of self-assembly offers a robust alternative to flow focusing for encapsulating multiple cores in a single coating film and the potential to build more complex colloidal building blocks by de-confining the clusters.

我们报告了微流通道的突然膨胀时，涂层液滴自组装成规则簇的观察结果。在通道扩张的上游形成了由规则的涂层微滴组成的双乳状液，因此，即使不是独立地施加不同的入口压力，也可以改变液滴间的距离，液滴长度，速度和涂层厚度。如果扩大的通道保持足够的封闭状态，以防止在双排中传播，则液滴可以在扩展时顺序组装成规则的线性簇。液滴通过其涂膜的聚结与前面的聚结团聚结在一起。当液滴在膨胀时彼此接近并达到临界距离时，就会发生这种合并，通过火车内的流体动力相互作用实现。获得了包含有限数量的液滴的簇，这是因为在聚结期间液滴组件的重新配置增加了到随后液滴的距离。减小墨滴间距离将群集大小增加到一个最大值，超过该最大值，将形成连续的群集。在一个简单的模型中将这些观察结果形式化可以发现任何大小的簇都是可能的，但是它们出现在参数值范围越来越窄的情况下。我们的实验观察结果表明，背景实验波动实际上限制了最大离散簇的大小。