Concentration Field Evolution during the Drying of a Thin Polymer Solution Film near the Contact Line

Abstract

An experimental study is performed for polymer concentration field measurements during the drying of an aqueous poly(vinyl alcohol) solution inside a shallow cavity near a vertical side wall. The measurements are based on optical techniques such as 3D confocal microscopy for laser-induced fluorescence analysis. The results reveal a significant concentration heterogeneity across the film near the meniscus during the drying process. The concentration at the solution-air interface remains higher compared to the bulk, and it increases toward the pinned contact line and also over time. A skin layer starts forming as the surface concentration reaches the glass-transition concentration, after which the evaporation rate starts decreasing. Regardless of the cavity depth and the initial polymer concentration, the drying film undergoes a similar concentration evolution during the evaporation process, although minor differences can be recognized. For instance, a low local capillary number at the surface is associated with a wavy surface concentration profile while at higher capillary numbers disturbances are damped and a much more uniform concentration profile is observed.

Evolution of entrained water film thickness and dynamics of Marangoni flow in Marangoni drying

“…The Marangoni number was introduced to investigate the competition behaviour of the Marangoni stress and the viscous force induced by the wafer withdrawn. 38,39 …”

Pursuing Phase Transitions of a Concentrated Polymer Solution by In Situ Fluorescence Measurements Based On Aggregation-Induced Emission

“…The fact reveals that enhanced emission is mainly attributed to the restriction of the molecule motion, which has nothing to do with aggregation. In contrast to traditional fluorescent probes, 8,24 the most evident feature of the AIE luminogen is its high sensitivity to the confinement extent of the surrounding microenvironment. Ultimately, charts of the fluorescence intensity vs the AIE concentration in pure water and a PVA aqueous solution were made for comparison, as further increased remarkably when the PVA concentration increased from 15% to 25%; at that moment, the TTVP concentration reached 12.5 mol/L.…”

“…The evolution trend of the mean concentration (C m ) of a PVA aqueous solution can be intuitively inspected by gravimetric analysis 12 or by measuring the height of the liquid film. 8 As C m increases, the PVA aqueous solution gradually loses its fluidity to a gel-like state with restricted chain dynamics. Since the water evaporation rate at the air−solution interface remains faster than that in the bulk, a concentration difference develops between the surface and core of the solution, i.e., skin−core delamination.…”

Flexible, Transparent, Thickness-Controllable SWCNT/PEDOT:PSS Hybrid Films Based on Coffee-Ring Lithography for Functional Noncontact Sensing Device

“…19−23 This results from the capillary flow and marangoni flow in the liquid drop during the drying process. 24 Normally, it can be an obstacle in numerous applications that require homogeneous solute deposition for film or hump accumulation for tracking, motivating researchers to intensively seek ways to control the CRE. The main contribution contains the particle shape (sphere to ellipsoids), 25 solution formula (solvent with high boiling point and low surface tension), 26,27 photosensitive surfactants as template, 28,29 and baking condition (low temperature to avoid CRE).…”

“…Coffee ring effect (CRE) is a very ubiquitous phenomenon, in which, when a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in a ring-like pattern. − This results from the capillary flow and marangoni flow in the liquid drop during the drying process . Normally, it can be an obstacle in numerous applications that require homogeneous solute deposition for film or hump accumulation for tracking, motivating researchers to intensively seek ways to control the CRE.…”