ICE | The Israel Chemist and Chemical Engineer | Issue 8

9 Scientific Article The Israel Chemist and Chemical Engineer Issue 8 · November 2021 · Kislev 5782 convection. With heat, the surface tension becomes lower, and the flow proceeds according to Marangoni convection. Drag forces act on the particles, which move along streamlines of the liquid (Figure 2a, left). While material accumulation following heating by optothermal means has been shown quite commonly, few researchers demonstrated assembly on substrates in a permanent manner. Such permanent fixing of dispersed materials may be achieved by making the continuous phase solid (e.g., hydrogel) by depletion forces or Van der Waals interactions. 2.1.2. Micro-bubble assisted printing A beamproduced by a continuous wave (CW) laser is absorbed by the particles or by a substrate that absorbs light. Due to the resultant heating, the pressure of the vapor increases until a microbubble is formed [6]. Focusing the laser close to the interface of the liquid and the substrate produces a gradient in temperature, such that the part of the microbubble closer to the substrate is hotter. This produces gradients in surface tension and density, which lead to convection currents (natural and Marangoni, respectively) that carry the particles, along with capillary forces. Some of the particles are pinned at the interface between the three phases (gas, liquid and solid) and show typical spherical deposition around the contact area of the bubble/substrate (Figure 2b) [7,8]. By moving the sample or the laser, the micro-bubble can propagate by depinning the bubble/liquid/substrate interface. Particles are deposited around the new location of the bubble, and micro-structures can be printed by repeating this process. Continuous patterns may be formed by laser modulations that enable improved control over the microbubble’s size and prevent its pinning to the deposited material [7]. 2.2. Optical forces Optical tweezers (OTs) are the best known method that uses optical forces to manipulate particles [9–14]. When the particle size is considerably greater than the wavelength of the laser (Mie regime), a force is generated in accordance with momentum conservation of absorbed, reflected or refracted photons. If the refractive index is higher than that of the medium, the force will pull/push it towards the most intense gradient. Nanoparticles (NPs) much smaller than the wavelength (Rayleigh regime) experience an electrostatic force due to different polarizability with respect to the medium. NPs with higher polarizability have a dipole moment that arises from the light’s electric field, and advance along the intensity gradients toward the focal point. Axial/radial components are parallel/perpendicular to the beam direction. Materials trapped with OTs may be micro-printed on substrates (Figure 2c) by movement of the trap with respect to the substrate. Interestingly, optical forces allow microprinting even without optical trapping. Material can be pushed towards the substrate using the component along the axis (e.g., for particles that are highly scattering). In either case (trapping/pushing), Van der Waals interactions can bind particles with the substrate. Particles may be permanently set on the substrate also by local thermal heating, gelation, electrophoretic deposition, and ultraviolet (UV) triggering. 3. Local directed synthesis Beyond particle/cell assembly, lasers allow localized synthesis from liquid/dissolved precursors. Advantages include greater stability and avoidance of preparation steps or stabilizers that can have a negative impact on the properties of the deposited material. Complex systems can be formed more easily, for example alloys, which are difficult to produce frommaterials that are preformed. 3.1. Single-photon reactions When a single photon excites an electron, thereby promoting a chemical reaction, photo-polymerization may occur in which laser irradiation polymerizes liquid or dissolved monomers. For liquids, reactions of single photons arise from laser illumination (mainly in the UV range) where photons can excite electrons (Figure 2d). The resin includes liquid molecules with a variety of functions – building blocks (monomers/cross linkers) and photo-sensitizers, which absorb light and transfer energy to photo-initiators, forming reactive species (e.g., radicals/cations) that initiate polymerization. These functions can be obtained in molecules of different type, or in separate parts of molecules of the same type. The desired 2Dmicro-structure is formed by steering the laser on a (thin) layer of the resin. Repeated scanning with an additional thin layer produces a 3D polymeric microstructure. The need for such layered 3D printing arises from the limited depth of penetration and selectivity in the perpendicular direction (along the z-axis). This technology is used commercially in many systems for a variety of applications. 3.2. Multi-photon reactions Non-l inear mult i-photon processes where elect ron excitation requires more than one photon (and a chemical reaction is promoted) proceed with rather low probability, as nearly simultaneous absorption is needed to initiate the process, requiring high energy pulsed lasers. The multiphoton absorption mechanism is used mainly for photopolymerization and reduction of metal ions. Usually, the liquid is transparent to the wavelength, obviating the need to add layers of thin liquid resin for 3D formation (required for single photon reactions). The substrate is moved relative