Suppression of coffee ring effect in high molecular weight polyacrylamide droplets evaporating on hydrophobic surfaces

In this work, we experimentally report the concentration and molecular weight dependence of the deposit patterns of polyacrylamide (PAM) droplets on hydrophobic surfaces. We find that with an increase in non-dimensional concentrations c/c* ranging from 0.16 (dilute) to 66.66 (semi-dilute entangled) there is a gradual transition from ring to non-ring patterns. However, with a decrease in the molecular weight of the polymer by one order, the coffee ring formation was not suppressed for the reported range of concentration. We attribute these results to the role played by the critical overlap concentration (c*) and diffusion coefficient of polymer along with the evaporation modes. The present study focuses on the drying of pure polymer droplets instead of the widely studied coffee ring effect in drying of droplets with rigid solid phase colloidal particles. We believe that these experimental findings would be of immense insight in applications such as inkjet printing, surface coatings, paints and detection assays in biotechnology for tuning polymer deposit patterns as a function of polymer molecular weight and concentration.