We investigate the effect of surfactant on the formation of nanoparticle aggregates that resulted from evaporation of sessile nanofluid droplets theoreticdly and experimentally. A Monte Carlo model is developed to explain the transition from the coffee-ring pattern to the uniform deposition in drying the pinned sessile nanofluid droplets. The model applies the diffusion limited cluster-cluster aggregation approach coupled with the biased random walk of nanoparticles. The experiments show that the addition of surfactant in nanofluids helps the formation of a coffee ring instead of the uniform domain coverage. The simulations suggest an explanation of this transition by controlling the sticlcing probability parameter between the particles. The simulated results statistically agree with the experimental observation of the finally dried graphite nanoparticle structures from the pinned nanofluid droplets.
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