Beyond the coffee-ring effect: Pattern formation by wetting and spreading of drops

Drying of colloidal drops on solid surfaces is the widely known method to form self-assembled patterns. The underlying principle of this method is the phenomenon known as the coffee-ring effect. Here, we report a phenomenon of pattern formation involving not drying but conversely wetting and spreading of drops on a solid surface containing a thin layer of dispersed particulates. Fascinating ringlike patterns are formed in a subsecond timescale by the interplay between the dynamics of spreading and imbibition. Occasionally, such patterns can be observed when, say, water spills on dusted floors and they are often misidentified as those formed by the coffee-ring effect. In the highly wetting scenario, we found that this pattern formation is independent of the liquid properties, however it is strongly dependent on the powder properties. Our findings have both fundamental and technological importance.

Figure 1

Patterns by drop spreading. (a) Example of ringlike stains formed when water drops spilled on a dusted floor. Impact resulted in large rings. (b) A ring formed by spreading of a gently placed ethanol drop (volume $2\mu \mathrm{L}$) on a thin layer of Nebasulf powder (white) on a glass slide. (c) Concentric rings formed by spreading of an ethanol drop (volume $1\mu \mathrm{L}$) on a layer of candle soot (dark) coated on a glass slide. The differences in the patterns in (a), (b), and (c) are related to the properties of the powder layer (see main text). (d) As the spreading proceeds, the initial large velocity of spreading, $u_s$, decreases and becomes comparable to the imbibition velocity averaged over the thickness of the layer, ${\overline{u}}_w$. At this stage, i.e., $r=r_o$, the contact line “sees” the underlying glass substrate.

Figure 2

(a) $r_o$ obtained from the images of the patterns, formed with $2\mu \mathrm{L}$ drops of silicone oil ($\star$), ethanol-glycerol mixture ($◯$), hexadecane ($\square$), and ethanol ($▿$). The solid line is a guide to the eyes. (b) $r_o$ vs the radius of the drop. Regardless of the liquid type or drop size, $r_o/R$ has a value $1.32\pm 0.05$. The error bars are the standard deviation of at least eight different drops.

Figure 3

Dynamics of pattern formation. (a) Dimensionless radius as a function of dimensionless time for drops of different liquids (shown in the panel). The solid line is a theoretical prediction by Eq. (1) (with $\epsilon =1.3$). Low-viscous drops deviate from this prediction (see the main text). $\epsilon =1.8$ on thin-layer chromatography (TLC) paper. On both soot and TLC, $r\left(t\right)$ is higher than that expected on a smooth surface ($\epsilon =1$). (b) The dimensionless velocity as a function of dimensionless radius. The inset is a zoom of the curves. The solid and dashed lines are the predictions from Eqs. (2) and (3) (with $a=25\mathrm{nm}$; $z_0=13\mu \mathrm{m}$), respectively.

Figure 4

Inset: The rings formed by an ethanol drop (volume of $2\mu \mathrm{L}$) on oxygen-plasma treated soot. The separation between the consecutive rings ${\lambda }_n$ vs $n$ for rings on the soot disk, $r<r_o$ ($▿$), and beyond the disk, $r>r_o$ ($▾$). The red filled and open circles represent the fitted values (see the main text). The error bars are the standard deviation for four drops.

Figure 5

Slow spreading of a drop of glycerol-ethanol mixture on a soot layer coated on PDMS resulted in the separation of individual rings to form microwires due to the decreased adhesion of soot and PDMS.