A drying droplet changes its morphological pattern depending upon complex pattern forming system. To control the distribution of solute particles in a droplet during drying is an important aspect in many scientific and industrial purposes. In this work, with the help of optical microscopy, we study characteristic patterns generated in dried drops of colloidal copper sulphate (CuSO 4 · 5H 2 O) solution on surface of glass. At lower concentration of salt solution the growth pattern follows a monofractal structure whereas at higher concentration, the selfassembled pattern gradually gets disappeared. Calculating the fractal dimension (FD) of the generated patterns by box counting method with help of imageJ, it is observed that the patterns resemble DLA structure through a specific range of concentration of the salt solution.

Patterns in drying droplets formed from colloidal solution of copper sulphate and gelatin are investigated with respect to variation of substrate hydrophobicity and salt concentration. Hydrophilic substrates as (i) glass, (ii) quartz and hydrophobic substrate as (iii) polypropylene (PP) have been used. It is observed that the dry residue pattern of salt crystals shows curved branches of crystalline aggregate growth about droplet centre for hydrophilic substrates, while thick, light and dark concentric bands of aggregates are observed for hydrophobic substrates. The geometry and topology of the patterns have been characterized through an analysis of fractal dimension and the topological measure, Euler characteristic. The fractal dimension of the deposit increases substantially with salt concentration for hydrophilic substrates, but decreases with concentration for hydrophobic substrate. Our analysis leads us to propose that an optimal viscosity contrast that facilitates prominent viscous fingers is a function of contact angle and salt concentration. We propose that substrate hydrophobicity and salt concentration together are responsible for DLA-like aggregation in evaporating droplets.