DNA nanotechnology offers many tools to assemble soft materials on the nano- mesoscale. Here we present results on DNA and colloidal assembly in the bulk and at an oil-water interface.
Using the binding specificity of DNA, we study the emergence of viscoelasticity in hydrogels made of DNA building blocks as they are cooled below the melting temperature, Tm, of their ‘sticky’, single-stranded (ss)DNA overhangs [1,2]. Using Diffusing Wave Spectroscopy for micro-rheology, we show that a 1:1 mixture of Y-shaped DNAs with complementary sticky overhangs remains in the fluid state well above the melting temperate of the overhangs. Below the melting temperature of the overhangs, the DNA undergoes a characteristic percolation transition at Tm, and 3D-hydrogels forms well below Tm [1,2]. The resulting elastic plateau modulus Gp dramatically increases when we remove the flexible joint between the Y-shapes. When going from a system of only Y-shapes to a mixture of Y-shapes and linear DNA linkers bridging them, we see an even more dramatic change in the network formation. In fact, by choosing the length of the fully flexible T-joint, we can tune the system from a gel network to a system that remains liquid at all temperatures with lower viscosity than at high temperatures.

Moreover, we used DNA tethers to study colloids anchored to a water-oil interface [3]. These DNA-anchored colloids are fully immersed in the water phase. Thus, they do not disturb the oil-water interface but allow the tethered colloids to diffuse freely along the oil-droplet surface without additional (optical) forces. Our combined experimental and theoretical analyses show that local temperature gradients induced by optical tweezers cause a thermophoretic force pushing the trapped particle towards the colder oil phase. The latter causes an attractive long-ranged hydrodynamic flow towards the laser focus, promoting out-of-equilibrium crystallization of the DNA tethered colloids around the trapped particle. The crystallization is enhanced by scattering forces known as optical binding.