Colloidal crystal formation using optothermal forces

We show that colloidal crystals can be assembled by means of temperature gradients produced by light absorption (λ=1070 nm) in a 21 nm titanium thin film deposited on one of the cell´s walls. Depending on the position of a 100x microscope objective focus within the 20 μm thick cell, three different regimes of crystal formation can be identified: 1) convective currents regime; 2) convective-thermophoresis regime, and 3) thermophoresis regime. We show that defects on the crystal can be modified dynamically by switching on and off the laser beam. In addition, the crystal can be 2D manipulated along the substrate. This technique could lead to the formation of large area colloidal crystals for photonics applications.