8 Scientific Article The Israel Chemist and Chemical Engineer Issue 8 · November 2021 · Kislev 5782 Figure 1. Common setup for laser guided printing from a liquid medium. Laser focused on the liquid forms 2D/3D microstructures by moving relative to a substrate carrying liquid. The various deposition mechanisms are represented by a question mark. Reproduced with permission [4] . Copyright 2021, Wiley-VCH. 2. Directed assembly of preformed materials Focused laser illumination can assemble preformed materials at the focal spot owing to thermal and/or optical force. 2.1. Thermal force Photo-therma l heat ing moves par t icles mainly by thermophoretic motion and convective flow. Micro-bubble assisted printing is a special case where the motion is convective, and particles are pinned to the microbubble base. 2.1.1. Photo-thermal printing Thermophoresis — particle motion due to a thermal gradient — proceeds mainly by an opto-thermoelectric mechanism. Local heating leads to a temperature gradient and ion separation according to the Soret coefficient. The consequential local electric field results in movement of charged particles towards (or away from) the heated area (Figure 2a, right). Photo-thermal printing may alternatively proceed by a mechanism in which the liquid motion carries particles. Local heating of the liquid can produce variations in pressure, density and surface tension. The density tends to decrease with temperature, as most solvents expand, resulting in flow driven by buoyancy, gradients in pressure, and local Figure 2. Methods of printing from a liquid according to material origin – preformed/synthesized locally. Preformed assembly methods are divided according to forces that affect material movement towards the focal spot. Local synthesis methods are divided according to the photon absorption mechanism. Reproduced with permission [4]. Copyright 2021, Wiley-VCH.
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