Figure 1

(a) The coffee-ring flow is induced by the divergence of the vapor flux $J$, where $J_0$ is the flux at the drop center. The Marangoni flow is induced by evaporative cooling and is directed inward along the liquid surface [$k_R\approx 10$ (the thermal conductivity ratio between the substrate and the liquid) for decalin on glass [26]. The capillary force effect is induced by contact between the outermost colloids and the liquid surface. The outermost particle position ($x_P$) and the stagnation point ($x_M$), where the direction of the Marangoni flow is inverted, are important for identifying the particle size effect. (b) Representative confocal microscopy images ($911\times 911{\mu \mathrm{m}}^2$) of the final deposition patterns from 0.5 μl drops of colloidal fluids on glass. Small colloids with $r_S=0.1$ μm and volume fractions ${\varphi }_S=0.1\%-1\%$ tended to form edge rings (Movies S1 and S2 [26]), whereas large colloids with $r_L=1.0$ μm and ${\varphi }_L=0.1\%-1\%$ formed bumps (Movies S3 and S4 [26]). Fingers were observed for bidispersed colloids (${\varphi }_S\approx 0.05$$\%$ and ${\varphi }_L\approx 1$$\%$). (c) Cross-sectional confocal image for a single finger, showing a finger thickness of ∼20 μm.Reuse & Permissions

Figure 2

For bidispersed colloids (${\varphi }_S\approx 0.05$$\%$ and ${\varphi }_L\approx 1$$\%$; Movie S5 [26]), sequential confocal images of a 0.5 μl decalin drop ($R_0\approx 1.2$ mm and ${\theta }_0\approx {20}^{\circ }$) on glass show that the small colloids formed the outer ring, whereas the large colloids formed the inner ring. The inner ring position ($x_P$), which is the average of the front and rear positions, gradually shifted to the center. Once the fingers were generated after $t/t_f>0.6$ (${\theta }_c=8.1^{\circ }$), they grew as the inner ring width increased by coffee-ring-driven particle deposition.Reuse & Permissions

Figure 3

(a) The evolutions of the front and rear positions of the inner ring show that the front grew continually whereas the rear halted after 1380 s ($t/t_f\approx 0.56$, $t_f=2465$ s). The ring width was defined as the difference between the front and rear positions. (b) The correlation between the ring width and the finger height suggests that the finger growth originated from the ring width, driven by the coffee-ring effect.Reuse & Permissions

Figure 4

Schematic phase diagram guides us to possible phase spaces for the ring, bump, and finger formations, with the particle volume fractions corresponding to large and small colloids (${\varphi }_L$ and ${\varphi }_S$, respectively) for a fixed droplet radius.Reuse & Permissions