On the influence of optically controlled thermocapillary flow during vertical convective assembly: Origin of remora disk-like patterns

Vertical convective assembly, a cost-effective and efficient colloidal assembly strategy, has garnered interest from a wide range of disciplines, including photonics and sensors. In this work, we reveal the role of nonuniform temperature distribution at the three-phase contact line (TPCL) during the vertical lifting of the substrate from the colloidal suspension. Conventionally, vertical assembly is performed under isothermal conditions, and the possible outcomes are uniform particle deposits and discrete lines based on stick-slip behavior. We demonstrate that exposing the TPCL with a nearly Gaussian-type temperature profile under an optimal lifting speed of 0.8–5 μm/s results in a new kind of particle pattern, which we call remora disk-like assembly, with periodic central thick regions and lamella-kind structures on either side. We generate the required temperature gradient by irradiating the TPCL with a laser beam, whose emission wavelength matches the plasmonic excitation of the nanoparticles used (λ = 532 nm). The nonuniform temperature distribution at the TPCL (ΔT = 13 °C) generates a corresponding thermocapillary flow, which drives the particles toward the TPCL in a gradient manner. We develop a physical model to explain the particle deposition mechanism, the nature of the remora disk assembly, and the asymmetric depinning behavior of the meniscus. Furthermore, by varying the lifting speed, we could tune the morphology and spacing of the patterns. We believe the new insights on the particle dynamics under optically controlled thermocapillary flow could significantly contribute to the fundamental understanding as well as enriching the applied aspects of the vertical lifting-based colloidal lithography.