Optothermal assembly and manipulation of colloids via non-coherent light

Abstract Soft-matter systems can be manipulated and driven far from equilibrium via optothermal manipulation, yet current approaches rely on focused lasers and complex optics. Here we introduce a microscope-integrated optothermal platform that uses incoherent wide-field illumination and a thin metal film on the substrate to generate controllable temperature gradients. Closing the condenser diaphragm of a commercial inverted microscope defines a tunable illumination zone whose absorbed power drives thermo-osmotic flows and directed particle transport. We quantify the resulting temperature profiles and flow fields using tracer tracking and continuum modeling, and connect them to the growth kinetics of light-induced colloidal crystals. We demonstrate the assembly of large, reconfigurable two-dimensional crystals without depletants, and the manipulation of colloidal crystals and flocks. This simple, low-power approach extends previous light-control of active colloids into a versatile, microscope-ready optothermal toolbox for out-of-equilibrium assembly and manipulation of soft-matter systems.