Sub-Newtonian coalescence dynamics in shear-thickening non-Brownian colloidal droplets.

Abstract

Recent investigations into coalescence dynamics of complex fluid droplets revealed the existence of sub-Newtonian behaviour in polymeric fluids (elastic and shear thinning). We hypothesize that such delayed coalescence or sub-Newtonian coalescence dynamics may be extended to the general class of shear thickening fluids. To investigate this, droplets of aqueous corn-starch suspensions were chosen and their coalescence in the sessile-pendant configuration was probed by real-time high-speed imaging. Temporal evolution of the neck (growth) during coalescence was quantified as a function of suspended particle weight fraction, <i>ϕ</i>_w. The necking behavior was found to evolve as the power-law relation, <i>R</i> = <i>at</i>^<i>b</i>, where <i>R</i> is the neck radius, with exponent <i>b</i> ≤ 0.5, implying that it is a subset of the generic sub-Newtonian coalescence. Furthermore, the coalescence dynamics could be demarcated into two distinct regimes, <i>b</i> ∼ 0.5 and <i>b</i> < 0.5, where the emergence of visco-elastic pinch-off response was observed in the latter regime. The particle fraction demarcating these regimes, designated as the critical particle weight fraction, <i>ϕ</i>_w ∼ <i>ϕ</i>_c > 0.35, also coincides with the existence of 'jamming' and 'flowing' regions within the neck during viscoelastic pinch-off of cornstarch suspensions (Roché <i>et al.</i>, Phys. Rev. Lett., 2011, <b>107</b>, 134503). We also propose a simplistic theoretical model that captures the observed delay in coalescence dynamics implicitly through altered suspension viscosity stemming from increased particle content.