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Assembling colloidal matter with an opto-thermophoretic strategy (w/video)

(Nanowerk Spotlight) Colloids are nanoscale particles that are finely distributed throughout a liquid. The suspension of nanoparticles has many uses – in foods, cosmetics, healthcare products, agrochemicals, and drug delivery systems. These nanoparticles are constantly in motion due to the principle of Brownian motion. Since they are electrically charged, colloids experience forces of attraction and repulsion that can be harnessed to control and manipulate their behavior.
Since colloidal particles exhibit collective behaviors beyond their individual properties when the nanoparticles are built into colloidal matter, researchers are keen to develop new strategies to rationally organize colloidal particles into complex structures for new functions and devices.
For example, colloidal matter, which consists of particles with sizes comparable to or below optical wavelength, has been explored as photonic crystals and metamaterials with unique properties such as optical bandgap and negative refractive index.
Previous assembly approaches, including self-assembly and electric/magnetic field directed assembly, have been developed to build diverse colloidal matters. These techniques feature high throughput but with limited structural configurations. Specifically, some of the techniques highly rely on the physical performance of the colloidal particles.
Researchers, led by Yuebing Zheng, Assistant Professor of Mechanical Engineering and Materials Science & Engineering at the University of Texas at Austin, now have developed a versatile colloidal assembly strategy – termed opto-thermophoretic assembly (OTA) – to build artificial colloidal matter in a wide range of colloidal materials, sizes, and shape.
They have reported their findings in Science Advances("Opto-thermophoretic assembly of colloidal matter").
Schematic illustration of the concept of opto-thermophoretic assembly (OTA)
Schematic illustration of the concept of OTA. (A) Dispersion of colloidal atoms in solvents. (B) CTAC surfactants adsorbed on the surface of the colloidal particles form molecular double layers, generating a positive and hydrophilic surface. Meanwhile, the CTAC molecules self-assemble into CTAC micelles with high surface charge density. (C) Manipulation of the colloidal atoms with the light-directed dynamic temperature field, and bonding of the closely positioned colloidal atoms with depletion attraction. (D) Construction of colloidal atoms into one-dimensional (1D) and 2D assemblies. (© AAAS) (click on image to enlarge)
"Using a light-directed thermoelectric field to capture and manipulate colloidal atoms and connect them with depletion attraction force, we achieved both low-power operation and tunable bonding length and strength in the colloidal matter," Dr. Linhan Lin, the paper's first author, tells Nanowerk. "The OTA approach releases the rigorous design rules required in the existing assembly techniques and enriches the structural complexity in colloidal matter, which will open a new window of opportunities for basic research on matter organization, advanced material design, and applications."
The OTA strategy can be widely applied to build different electronic and photonic colloidal devices. Specifically, interaction of colloidal particles with sizes comparable to or smaller than the wavelength of light exhibit unique optical performance.
Specific potential applications of the OTA approach are:
  • Colloidal chiral materials – building colloidal superstructures with tunable and reconfigurable chiral response;
  • Structured color – controlling the interaction between wavelength scale colloidal particles to direct light propagation in the materials;
  • Second harmonic generation – building hybrid colloidal structures with both active and passive colloids with tunable interaction.
  • The understanding of colloid science and opto-thermo-fluidics in the OTA strategy paves the way toward precisely tuning and reconfiguring the structures and properties of colloidal matter.
    Light-driven thermophoretic trapping, manipulation, and assembly of colloidal particles into colloidal superstructures. (Video: Zheng Research Group)
    "We can foresee diverse applications of OTA in fabrication of photonic structures with designer optical performance such as optical chirality, second harmonic generation, and so on," notes Lin.
    He adds that future research in colloidal assembly will focus on the controllability as well as tunability of colloidal superstructure to achieve diverse electrical and optical functions. Further advanced assembly techniques will be developed to facilitate the fabrication of colloidal devices for on-chip applications. Potential challenges in this area include 3D manipulation and organization of colloidal particles, as well as pushing down the size limit of colloidal particles in the assembly.
    "The versatile construction of optical matter with enriched structures and reconfigurability, in combination with real-space imaging of the motions of individual particles by optical microscopy, will also advance the understanding of the general principles of self-assembly," concludes Zheng. "Eventually, the scale-up of the directed assembly for a large quantity of designer structures and materials will lead to a wide range of technical applications of colloidal matter."
    By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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