Numerous living organisms as well as artificially created self-propelled objects can form dissipative structures due to the nonlinear effects and nonequilibrium of the system. Here we present an active oil-in-water emulsion in which the oil droplets take part in the reciprocating motion under the action of Marangoni flow near the air–water interface. The droplet dynamics in the emulsion is governed by the chemical reaction proceeding between quiescent copper particles and ammonia and by the convective mixing of a surfactant. We established that the reciprocating motion of droplets in the emulsion arises as a result of a periodic change in the Marangoni flow direction at the air–water interface. The feature of the considered system is that the reciprocating motion of droplets is realized only when the surface area fraction of droplets in the emulsion is close to the density of a two-dimensional colloid crystal. Oscillations degenerate under the reduction in surface area fraction to the critical value of ∼50% since the existence of oscillations in the emulsion requires a suppression of the surfactant convective mixing between the inner layers of liquid film and the air–water interface.

中文翻译：

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空气-水界面附近乳液中油滴的振荡运动

由于系统的非线性效应和非平衡性，许多生物体以及人工创造的自推进物体可以形成耗散结构。在这里，我们提出了一种活性水包油乳液，其中油滴在空气 - 水界面附近的 Marangoni 流的作用下参与往复运动。乳液中的液滴动力学受静态铜颗粒和氨之间的化学反应以及表面活性剂的对流混合控制。我们确定乳液中液滴的往复运动是由于空气 - 水界面处 Marangoni 流动方向的周期性变化而产生的。所考虑系统的特点是只有当乳液中液滴的表面积分数接近二维胶体晶体的密度时，才能实现液滴的往复运动。在表面积分数减少到~50% 的临界值下，振荡退化，因为乳液中振荡的存在需要抑制液膜内层和空气 - 水界面之间的表面活性剂对流混合。