Controlling and characterising the deposits from polymer droplets containing microparticles and salt.

Y Msambwa,F F Ouali,A S D Shackleford,D J Fairhurst

doi:10.1140/epje/i2016-16021-x

Y Msambwa, F F Ouali + Show 2 more

Open Access

https://doi.org/10.1140/epje/i2016-16021-x

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Abstract

A coffee ring-stain is left behind when droplets containing a wide range of different suspended particles evaporate, caused by a pinned contact line generating a strong outwards capillary flow. Conversely, in the very peculiar case of evaporating droplets of poly(ethylene oxide) solutions, tall pillars are deposited in the centre of the droplet following a boot-strapping process in which the contact line recedes quickly, driven by a constricting collar of polymer crystallisation: no other polymer has been reported to produce these central pillars. Here we map out the phase behaviour seen when the specific pillar-forming polymer is combined with spherical microparticles, illustrating a range of final deposit shapes, including the standard particle ring-stain, polymer pillars and also flat deposits. The topologies of the deposits are measured using profile images and stylus profilometery, and characterised using the skewness of the profile as a simple analytic method for quantifying the shapes: pillars produce positive skew, flat deposits have zero skew and ring-stains have a negative value. We also demonstrate that pillar formation is even more effectively disrupted using potassium sulphate salt solutions, which change the water from a good solvent to a theta-point solvent, consequently reducing the size and configuration of the polymer coils. This inhibits polymer crystallisation, interfering with the bootstrap process and ultimately prevents pillars from forming. Again, the deposit shapes are quantified using the skew parameter.

Highlights

The work of Deegan et al [1] first thoroughly investigated the properties of the coffee ring-stain, commonly seen when suspension droplets are left to evaporate on a solid surface. They proposed a simple explanation for these deposits with just two requirements: firstly, the triple line at the edge of the droplet must remain pinned to the substrate throughout the drying process, known as constant contact radius drying (CCR) [2]; secondly the evaporative flux over the droplet varies with radius r measured from the centre of the droplet and diverges at the contact line r = R following a power law
Steep, tall pillars are seen at high polymer concentrations
We have shown particles disrupt pillar formation and polymer disrupts ring-stain formation

Summary

Introduction

The work of Deegan et al [1] first thoroughly investigated the properties of the coffee ring-stain, commonly seen when suspension droplets are left to evaporate on a solid surface They proposed a simple explanation for these deposits with just two requirements: firstly, the triple line at the edge of the droplet must remain pinned to the substrate throughout (most of) the drying process, known as constant contact radius drying (CCR) [2]; secondly the evaporative flux over the droplet varies with radius r measured from the centre of the droplet and diverges at the contact line r = R following a power law. In drying droplets of both dextran [11] and bitumen [12] the contact line becomes pinned, a flexible glassy skin with fixed surface area forms and as evaporation continues the film buckles leaving a final deposit in the shape of a sombrero

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