Manipulating colloidal residue deposit from drying droplets: Air/liquid interface capture competes with coffee-ring effect

- Lagrangian particle tracking model of colloids within drying droplets.
- Relative roles of air/liquid vs. solid/liquid interfaces on the coffee ring effect.
- Critical line separates colloids captured first by free interface or by substrate.
- The descending air-liquid interface could capture a large portion of colloids.
- Regulating dried deposit by tuning the colloidal forces with air-liquid interface.

Significant efforts have been undertaken in recent years to understand, and ultimately control, the drying patterns of colloidal particle filled droplets on solid substrates. The ubiquitous coffee-ring effect has long been attributed to the drag of radial liquid flow towards the droplet edge, driven by diffusive evaporation. Here we report new results showing that there is a temporal competition between migrating colloidal particles, dispersed randomly inside a droplet, and the air/liquid interface in reaching the solid surface. Using the Lagrangian modelling approach to track the motion of colloidal particles within the evaporating droplets, we reveal a boundary line that separates colloidal particles into two groups: above it colloids are captured by the air/liquid interface prior to reaching the solid surface and vice versa. This critical line allows us to quantify the effectiveness of the two emerging routes for manipulating the coffee-ring effect, which work by controlling the interaction between colloids and either the substrate or the air-liquid interface. Our modelling results affirm recent experimental evidence that the air-liquid interface of sessile droplets plays a vital role in the formation of residual deposits. This improved fundamental understanding provides versatile strategies to control droplet deposition morphology, especially the most desired uniform deposit as well as an unusual “overhang” shape of the cross section of the ring-like deposits.

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