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

Abstract

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.

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

“…Thermocapillary convection is generated in a thin suspension layer by a local heat source in a substrate, resulting in the transfer and redistribution of particles in the system. ,− ,, The axisymmetric surface tension gradient generates shear stress flow in the bulk layer, causing the liquid to move away from the heated zone and resulting in a decrease in the layer thickness in the heating zone . Due to negative capillary pressure below the curved surface, reverse bottom flow toward the heated zone is generated.…”

“…Figure displays the patterns resulting from the spatial distribution of particles in the three types of cells for suspension layer thicknesses in the central part of the cell of approximately 3 d p and 10 d p . The pattern deposit geometry was anticipated to be determined solely by geometry of the temperature and the flow fields, based on previously published works ,, and unpublished studies with line and cross-shaped heat sources. However, Figure shows that the geometry of the cell boundaries plays a crucial role in shaping the final patterns.…”

“…We have also shown the possibility of creating in this way circular assemblies of different areas and particle distribution densities 40,41 and ring-shaped assemblies of different diameters and ring widths. 42 According to the analysis of research studies, 37,40,41 the geometrical shape of the pattern is determined by the shape of the thermal field on the substrate or liquid surface. If the temperature field is axisymmetric, the resulting thermocapillary flows will also be axisymmetric, resulting in a pattern that is close to circular or ring-like in shape when the direction of the temperature gradient changes.…”

“…Previously, − we proposed a fairly flexible method based on a thermocapillary mechanism that allows dynamic control of the assembly of micron-sized particles by changing the magnitude and sign of an axisymmetric temperature gradient created by a local heater or heat sink in the substrate. We have also shown the possibility of creating in this way circular assemblies of different areas and particle distribution densities , and ring-shaped assemblies of different diameters and ring widths . According to the analysis of research studies, ,, the geometrical shape of the pattern is determined by the shape of the thermal field on the substrate or liquid surface.…”

Laser-Generated Scholte Waves in Floating Microparticles

“…For PS40 and PS80, shown in Figure 3 b,c, respectively, the rings inside the microparticle can be observed, and in the center of the microparticle lies the core, which is surrounded by the shell. Studies have shown that the structure inside the microparticle depends on the method of their preparation [ 25 , 26 , 27 ]. The area shown in Figure 3 is 0.3 mm × 0.3 mm in x - and y -direction with a pixel size of 2 µm.…”