Monolith formation and ring-stain suppression in low-pressure evaporation of poly(ethylene oxide) droplets

Abstract

When droplets of dilute suspensions are left to evaporate the final dry residue is typically the familiar coffee-ring stain, with nearly all material deposited at the initial triple line (Deegan et al, Nature, vol. 389, 1997, pp. 827-829). However, aqueous poly(ethylene oxide) (PEO) droplets only form coffee-ring stains for a very narrow range of the experimental parameters molecular weight, concentration and drying rate. Instead, over a wide range of values they form either a flat disk or a very distinctive tall central monolith via a four-stage deposition process which includes a remarkable bootstrap-building step. To predict which deposit will form, we present a quantitative model comparing the effects of advective build-up at the triple line to diffusive flux and use this to calculate a dimensionless number /. By experimentally varying concentration and flux (using a low-pressure drying chamber), the prediction is tested over nearly two orders of magnitude in both variables and shown to be in good agreement with the boundary between disks and monoliths at x ~ 1 -6.

How Robust Is the Ring Stain for Evaporating Suspension Droplets?

“…Samples were placed inside a cylindrical vacuum chamber (diameter 8.6 cm, height 5.4 cm) and connected through a valve to a Cole Parmer MD 4NT vacuum pump (Baldwin et al, 2012). The pressure was recorded using a transducer (KJLC 902) interfaced with controller and readout (KPDR 900) to provide accurate control over the pressure within the chamber.…”

Imaging internal flows in a drying sessile polymer dispersion drop using Spectral Radar Optical Coherence Tomography (SR-OCT)

“…Here η and r are the viscosity of the fluid and radius of particles in model paint, respectively. High Péclet numbers refer to rapid evaporation [29].…”

The effects of molecular weight, evaporation rate and polymer concentration on pillar formation in drying poly(ethylene oxide) droplets

“…In a 1 dimensional model [21,22] pillar formation was shown to be controlled by a Péclet type ratio of the evaporation rate, −V (where V is droplet volume), which drives the polymer to the contact line during pinned drying, and the gradient diffusion coefficient of the polymer, D G , which in the absence of evaporation would homogenize the solution, along with initial droplet parameters of mass concentration c 0 , droplet base radius R, contact angle θ. In this work, as supported by various theoretical modelling approaches to particle dynamics during sessile droplet drying [1,10,23], we assume that the majority of evaporation over the surface of a droplet occurs in a narrow wedge very close to the contact line.…”