ENTROPY DRIVEN AGGREGATION OF CNT IN A DRYING DROP ON HYDROPHILIC AND HYDROPHOBIC SUBSTRATE
Functional materials with desired properties can be assembled from colloidal particles of the necessary materials. However nanoparticles usually exist in a randomly aggregated state, which makes them difficult to separate and organize. The most important and difficult task, at the same time less developed, is not the production of nanoparticles but the assembly of a macroscopic material from them. This study is dedicated to the aggregation of carbon nanotubes by depletion forces and the effect of the hydrophobic and hydrophilic substrate. The process has been realized in a drying droplet of distilled water with colloidal particles and observed with an optical microscope. Colloid SiO<sub>2</sub> particles were used as a macromolecule solution causing the depletion forces. Long carbon nanotubes were chosen because they unlike the short ones show no reaction with SiO<sub>2</sub>. Nanotubes were ultrasonicated in distilled water for 5 min to obtain a colloidal mixture. The rate of evaporation of the droplet was controlled using analytic scales. It has been shown that the process of aggregation strongly depends on the substrate material. On a hydrophilic substrate the droplet cannot change its area and evaporates at constant speed. The strong capillary flows cause high orientation of nanotubes and organize them into transparent wires. By the contrast on a hydrophobic substrate the droplet shrinks steadily and the rate of evaporation decreases with constant rate. In this case no signs of self organization were detected. As a concentration increases the aggregates of nanotubes that are distributed in a droplet grow but they cannot escape the droplet and deposit on the substrate. As a droplet shrinks steadily all the components move with it and gather at the center creating a clot. The results may be used in production of functional and combined materials and as a method of segregation of colloids.