Fig. 5: Growth kinetics of spherulites in evapora mixed salt solution.
From: Controlled spherulitic crystal growth from salt mixtures
a Spherulite size L as a function of the square root of time t at RH = 50%. Each dataset is for one spherulite as shown in the photographs on the right: spherulites 1 and 2 are close to the contact line while 3, 4 and 5 are closer to the center of the droplet. The linear trend indicates that the growth of the spherulites is controlled by diffusion, with spherulites further away from the edge of the droplet growing faster than those near the center (indicated by slope Δ). The inset plot: the linear relation between local solvent viscosity η and the inverse diffusivity 1/D. A 12-fold viscosity increase is estimated for spherulite 5, and a significantly higher viscosity increase (70-fold) near the droplet’s edge (spherulite 1). b Rheological measurements of viscosity as a function of concentration with different molar Mg factions x. All measurements at T = 21 ± 1 ∘C. While the viscosity η of the pure sodium sulfate and pure magnesium sulfate solution are in the same order of magnitude as the viscosity of water, the viscosity of the xMg = 0.12 solution as a function of the saturation follows an exponential trend. The dashed line indicates \(\eta =4\times 1{0}^{-5}\exp (10.6{\beta }_{{{\mathrm{Mg}}}})\) with \({\beta }_{{{\mathrm{Mg}}}}={m}_{{{\mathrm{Mg}}}}/{m}_{{{\mathrm{Mg}}}}^{* }\) (R2 = 0.88). The viscosity is measured up to a saturation of βMg = 0.73. By then, the viscosity is about 65 times higher than the viscosity of water. The exponential fit predicts that the viscosity is increased to 111 Pa ⋅ s when spherulitic growth is initiated at βMg = 1.4 (dotted line).
