Near-Infrared Photothermal Microgel for Interfacial Friction Control

In the field of tribology, how to control the interfacial friction has evolved to be an intriguing research topic. Over the past decade, stimuli-responsive materials, either as lubricating additives or as modified layers, have been widely used to realize the interfacial friction control. Here, near-infrared (NIR) photothermal microgels (PTMGs) were developed as a kind of smart lubricating additive to regulate the interfacial friction. This composite microgel, poly(N-isopropylacrylamide-co-acrylic acid)@Au [P(NIPAM-co-AA)@Au], possesses a thermosensitive gel shell and a NIR photothermal core. Below the lower critical solution temperature (LCST), this composite microgel was hydrophilic and could be connected with water molecules through hydrogen bonds to form a hydrated layer, which played a role of hydration lubrication. When the microgels were irradiated by NIR light, the core was heated to elevate the temperature of the gel shell, which subsequently underwent a hydrophilic/hydrophobic transition surrounding the LCST. At this time, the composite microgel became hydrophobic, the hydrated layer disappeared, and the hydration lubrication effect did not work. That is to say, the interfacial friction can be modulated reversibly from a low level to a high level by using PTMGs as lubricating additives under the effect of NIR irradiation. Looking beyond, these kind of microgels, which were capable of modulating interfacial friction, show great potential in the design of smart sensors, controllable transmission, and bionic manipulators.