Assembling colloidal particles with sizes at or below the wavelength of light into unique structures such as photonic crystals and metamaterials can be achieved with electrostatic, hydrophobic, or other attraction/repulsion mechanisms or by adding additional materials. However, it only produces the desired assembly shape if the colloidal particles possess the optimum optical, magnetic, or electrical properties that respond to the internal or external applied forces. To expand the boundaries of possibility for colloidal matter assembly, researchers at the University of Texas at Austin, led by Yuebing Zheng, have developed an optothermophoretic assembly (OTA) method that instead uses an ionic surfactant (cetyltrimethylammonium chloride, or CTAC) to manipulate and assemble most any colloidal matter using a light-controlled temperature field.
The surfactant molecules create positively charged colloidal particles after their adsorption on the particle surfaces. The thermophoretic migration of the colloidal particles along the temperature gradient confines the particles at the hot laser spot when low-power continuous-wave laser light is irradiated onto a gold thin film that converts photon energy to thermal energy. In fact, the required energy of 0.8 mW/μm2 is 2–3 orders of magnitude smaller than optical tweezers. The green laser source is easily split with a digital micromirror device into any optical pattern that traps the colloidal particles in parallel with precise orientation control, assembling them into a predefined pattern. The thermal process further depletes the CTAC micelles with an osmotic pressure outside the depletion region exerted on the colloidal particles, leaving the colloidal particles bonded by their own attractive forces when the laser is turned off. Reference: L. Lin et al., Sci. Adv., 3, 9, e1700458 (Sep. 8, 2017).
