Innovations in exploiting photo-controlled Marangoni flows for soft matter actuations

The generation and control of microscale flows are crucial for fundamental as well as applied aspects of microfluidics. Commonly employed strategies for creating microflows are based on electric field, magnetic field, surface tension, temperature, pressure, light, etc. Among them, light as an external stimulus is gaining increased attention as it offers non-contact actuation, high spatial and temporal resolution, tunable wavelength and intensity, ease of miniaturization, and fast response. Optically tuning the surface tension is promising because a surface tension gradient of a few mN m^-1 along the liquid surface is sufficient to create a strong Marangoni flow. This can be done by either optically heating (thermocapillary) or modulating the chemical composition (soluto-capillary) at the liquid surface, which could be exploited to realize the transport/assembly of colloidal particles, droplets, bubbles and liquid marbles. In this perspective article, we focus on the innovative approaches in modulating surface tension using light, in the context of soft matter and microfluidic applications. First, we discuss the photo-controlled Marangoni flow-driven patterning and assembly of colloidal particles in different microfluidic systems, such as liquid droplets and liquid films. Next, we review the various methods in which the photo-controlled Marangoni flows are being exploited for the actuation of liquid droplets and liquid marbles over solid as well as soft substrates. Finally, we highlight the recent trends in optically controlled flow-based reconfigurable optics and energy harvesting systems.