Effect of suspended particles on spreading of volatile droplet on solid substrate

Abstract

We have been interested in behaviors of suspended particles in a volatile droplet placed on a smooth substrate. It is known that the particles gather and deposit in the vicinity of the macroscopic contact line of the droplet, which is generally called 'coffee stain problem'. A convective flow induced by nonuniform evaporation through the interface brings suspended particles toward the pinned contact line in the drying droplet, which forms a ring stain. We have focused on the dynamics of the droplet with/without suspended particles spreading on the solid substrate and on the behaviors of particles in the evaporating droplet. Spreading process of the droplet is significantly affected by the suspended particles. We indicate flow patterns in the droplet, in which the flow exhibits a modal structure with a mode number in the azimuthal direction, and indicate particles depositions after the dryout of the droplet. Three-dimensional particle tracking velocimetry is applied to reconstruct such unique flow patterns in the spreading process of the droplet. Resultant patterns of the particles depositing on the substrate are introduced.

Temperature-Dependent Electrical Properties of Graphene Inkjet-Printed on Flexible Materials

“…The coffee-ring structure was similar to what has been observed in various inkjet-printed materials. 7,8 As a result of pinning at the edge of the low contact angle area of the droplet, most GO sheets appeared to stack and form aggregated structures 5−10 nm high and 100−200 nm wide at the perimeter (Figure 3b). Interestingly, we consistently observed a "star"-shaped assembly of nanoscale features at the center region of the droplet (Figure 3c), while leaving a significantly lower number of sheets scattered between the center and the perimeter regions.…”

Colloidal Shape Effects in Evaporating Drops

“…A drop of evaporating water is a complex, difficult-to-control, non-equilibrium system. Along with capillary flow, the evaporating drop features a spherical-cap-shaped air-water interface and Marangoni flows induced by small temperature differences between the top of the drop and the contact line [1,2]. Thus, to understand the coffee-ring effect, one must understand pinning effects, fluid dynamics, particle-substrate interactions, substrate-fluid interactions, and more.…”

“…The interaction potential between two particles is related to the excess surface area created by these deformations [32]. For a single particle (i), the deformation energy is U i = γδA i , where δA i is the excess surface area due to interfacial deformation, which is proportional to the deformation size squared, i.e., δA i ∝ ∆h 2 The interaction energy of two particles (i, j) is…”