Transport and assembling microparticles via Marangoni flows in heating and cooling modes

Controlling Spatial Morphology of Microparticle Deposits via Thermocapillary Flows: Effect of Boundary Geometry

The production of particle deposits with a desired distribution geometry has significant potential for materials science, printing, and coating technologies. Most methods for achieving well-defined assemblies rely on the spontaneous evaporation of colloidal solutions on substrates with predetermined properties, or on precise control of particle arrangement by external stimuli. Here, we present a combined method that enables the production of centimeter-scale microparticle deposits with a desired geometric shape. The method is based on controlling the massive transport of microparticles by thermocapillary flow in a layer of volatile liquid in a cell with borders of the desired geometry. Capillary forces cause the liquid to be distributed in the cell, forming corner wetting menisci and the flat layer in the central area. The formation of particle deposits occurs in two stages, determined by the flow regime. At the initial stage, the axisymmetric thermocapillary flow occurs in the flat part of the layer, resulting in the circular shape of the particle deposit. During the transition to the second stage of assembling thermocapillary flow is localized in the corner wetting menisci that results in reshaping the current particle deposit to match the geometry of the cell borders. Here, we demonstrated the creation of circular, square, and triangular shapes of the patterns of polystyrene microparticles using a point heater located at the geometric center of the cell. The proposed method is reliable, easy to implement, and potentially capable of producing a wide variety of deposit geometries, making it an attractive technique for patterning and modifying surface properties with particles of any type.

Fabrication of Ring-Shaped Deposits of Polystyrene Microparticles Driven by Thermocapillary Mechanism

Fabrication of ring-shaped deposits of microparticles on solid surfaces with the desired length scales and morphology of particle arrangements is of great importance when developing modern optical and electronic resonators, chemical sensors, touch screens, field-emission displays, porous materials, and coatings with various functional properties. However, the controlled formation of ring-shaped patterns scaling from a few millimeters up to centimeters with simultaneous control of particle arrangement at the microscale is one of the most challenging problems in advanced materials science and technology. Here, we report a fabrication approach for ring-shaped structures of microparticles on a glass surface that relied on a local thermal impact produced by the subsurface heater and heat sink. Thermocapillary convection in the liquid covering microparticles in combination with evaporative lithography is responsible for the particle transport and the assembling into the ring-shaped patterns. An advantageous feature of this approach is based on the control of thermocapillary flow direction, achieved by changing the sign of the temperature gradient in the liquid, switching between heating and cooling modes. That allows for changing the particle transfer direction to create the ring-shaped deposits and dynamically tune their size and density distribution. We have studied the influence of the power applied to the heat source/sink and the duration of the applied thermal field on the rate of the ring fabrication, the sizes of the ring and the profile of the particle distribution in the ring. The proposed method is flexible to control simultaneously the centimeter scale and microscale processes of transfer and arrangements of particles and can be applied to the fabrication of ring structures of particles of different nature and shape.