Marangoni flow, a surface shear flow, is a promising option for propulsion when controlling liquid droplet motion on solid substrates. However, the applicability of Marangoni flow induced by heating the substrate for droplet manipulation is limited by low precision of motion control and a narrow range of substrate types. Herein, a novel noncontact light-driven droplet manipulation method by using photothermally active droplets is introduced. Marangoni flow is induced by local photothermal heating via near-infrared (NIR) irradiation resulting in an internal flow that drives the droplets. The photothermally active droplets slide away from the NIR light and the direction of motion can be precisely controlled by changing the irradiation position remotely. In addition, it is demonstrated that the addition of a miscible liquid to the droplets can reverse the direction of motion. Moreover, the authors show that spherical droplets on conventional liquid repellent surfaces move through a rolling mechanism instead of sliding. It is believed that this droplet manipulation method can provide a general way of droplet transportation on solid surfaces.

lubricant on the LIS is a fluid, substrates for the LIS should possess a porous or textured structure to induce the capillary force that retains the lubricant layer. In addition, the substrate must have a hydrophobic surface with a low surface energy to achieve a stable lubricant layer; the low surface energy is crucial since the total interfacial energy among substrate, liquid drop, and lubricant should be lower than that between substrate and liquid drop for effective repulsion of the liquid drops. [14,15] In line with the above requirements, numerous engineering technologies have been explored to fabricate slippery LISs. [16-18] However, their porous/textured structure results in weakened mechanical properties, thereby restricting fabrication to limited structures and materials. There has been some progress toward producing slippery surfaces on flat or nontextured surfaces. Recently, a stable lubricant-impregnated surface on a smooth substrate was introduced with the aid of π-electron interactions between the surface-modifying molecules and lubricant. [19] On this surface, phenolic groups were grafted onto the substrate, following which oleic acid was infused. The attractive interaction of the-OH group in oleic acid and π-electrons in the phenolic group increased the stability of the infused oleic acid on the surfaces, providing durability in extreme environments. [20] Even though this slippery surface does not require a porous or textured structure, the surface modification relies on the speci fic chemical interactions. This highly specific nature is a roadblock for the application of this method to other materials. A photo catalytically active lubricant-impregnated surface (PALIS) is a viable alternative for flat or nontextured slippery surfaces because the hydrophobized metal-oxide surface with a polydimethylsiloxane (PDMS) brush helps to form a stable PDMS lubricant layer. [21] However, the PALIS can only be prepared on a metal-oxide photocatalyst under UV light. [22,23] Most recently, a surface functionalization method to coat flat surfaces with PDMS brushes, followed by silicone oil infusion, was reported. To generate the PDMS brush, the surface was first treated with 1,3,5,7-tetramethylcyclotetrasiloxane, then immersed into a vinyl-terminated PDMS solution with a catalyst to graft PDMS by the hydrosilylation reaction. [24] Though the surface exhibited good liquid repellency, the reaction, including the pretreatments, was complicated and cumbersome.

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