Fast and programmable transport of droplets on a substrate is desirable in microfluidic, thermal, biomedical, and energy devices. Photoresponsive surfactants are promising candidates to manipulate droplet motion due to their ability to modify interfacial tension and generate “photo-Marangoni” flow under light stimuli. Previous works have demonstrated photo-Marangoni droplet migration in liquid media; however, migration on other substrates, including solid and liquid-infused surfaces (LIS), remains an outstanding challenge. Moreover, models of photo-Marangoni migration are still needed to identify optimal photoswitches and assess the feasibility of new applications. In this work, we demonstrate 2D droplet motion on liquid surfaces and on LIS, as well as rectilinear motion in solid capillary tubes. We synthesize photoswitches based on spiropyran and merocyanine, capable of tension changes of up to 5.5 mN/m across time scales as short as 1.7 s. A millimeter-sized droplet migrates at up to 5.5 mm/s on a liquid, and 0.25 mm/s on LIS. We observe an optimal droplet size for fast migration, which we explain by developing a scaling model. The model also predicts that faster migration is enabled by surfactants that maximize the ratio between the tension change and the photoswitching time. To better understand migration on LIS, we visualize the droplet flow using tracer particles, and we develop corresponding numerical simulations, finding reasonable agreement. The methods and insights demonstrated in this study enable advances for manipulation of droplets for microfluidic, thermal and water harvesting devices.

中文翻译：

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使用光响应表面活性剂动态操纵液体注入表面上的液滴

在微流体、热、生物医学和能源设备中，需要在基板上快速且可编程地传输液滴。光响应表面活性剂是操纵液滴运动的有希望的候选者，因为它们能够改变界面张力并在光刺激下产生“光马兰戈尼”流。先前的工作已经证明了液体介质中的光马兰戈尼液滴迁移；然而，在其他基材上的迁移，包括固体和液体注入表面（LIS），仍然是一个突出的挑战。此外，仍然需要光-马兰戈尼迁移模型来识别最佳光开关并评估新应用的可行性。在这项工作中，我们演示了液体表面和 LIS 上的 2D 液滴运动，以及固体毛细管中的直线运动。我们合成了基于螺吡喃和部花青的光电开关，能够在短至 1.7 秒的时间尺度内实现高达 5.5 mN/m 的张力变化。毫米大小的液滴在液体上的迁移速度高达 5.5 毫米/秒，在 LIS 上的迁移速度为 0.25 毫米/秒。我们观察到快速迁移的最佳液滴尺寸，并通过开发缩放模型来解释这一点。该模型还预测，表面活性剂可以使张力变化和光切换时间之间的比率最大化，从而实现更快的迁移。为了更好地理解 LIS 上的迁移，我们使用示踪粒子可视化液滴流，并开发相应的数值模拟，找到合理的一致性。本研究中展示的方法和见解使微流体、热和水收集装置的液滴操纵取得了进展。