Evaporation-controlled dripping-onto-substrate (DoS) extensional rheology of viscoelastic polymer solutions

Abstract

Extensional flow properties of polymer solutions in volatile solvents govern many industrially-relevant coating processes, but existing instrumentation lacks the environment necessary to control evaporation. To mitigate evaporation during dripping-onto-substrate (DoS) extensional rheology measurements, we developed a chamber to enclose the sample in an environment saturated with solvent vapor. We validated the evaporation-controlled DoS device by measuring a model high molecular weight polyethylene oxide (PEO) in various organic solvents both inside and outside of the chamber. Evaporation substantially increased the extensional relaxation time [Formula: see text] for PEO in volatile solvents like dichloromethane and chloroform. PEO/chloroform solutions displayed an over 20-fold increase in [Formula: see text] due to the formation of an evaporation-induced surface film; evaporation studies confirmed surface features and skin formation reminiscent of buckling instabilities commonly observed in drying polymer solutions. Finally, the relaxation times of semi-dilute PEO/chloroform solutions were measured with environmental control, where [Formula: see text] scaled with concentration by the exponent [Formula: see text]. These measurements validate the evaporation-controlled DoS environment, and confirm that chloroform is a good solvent for PEO, with a Flory exponent of [Formula: see text]. Our results are the first to control evaporation during DoS extensional rheology, and provide guidelines establishing when environmental control is necessary to obtain accurate rheological parameters.

Figure 1

Schematic of the evaporation-controlled DoS instrument with sacrificial solvent reservoir and environmental control chamber, adapted from Lauser et al. (see SI.2); set-up adapted from Dinic and Sharma. To perform DoS measurements, the syringe pump is used to extrude a droplet from a needle with radius $R_0$, which then forms a liquid bridge with height h upon contact with the substrate. A high-speed camera captures the evolution of the liquid bridge minimum radius R as the bridge thins in time. The environmental control chamber provides an atmosphere enriched in solvent vapor around the drop to prevent evaporation during this thinning process.

Figure 2

Representative evaporation-controlled DoS capillary thinning measurements of 3 mg/mL PEO in: (a) water, (b) NMF, (c) DCM, and (d) chloroform. Insets in (b) and (d) show 2D images of the thinning inside of the chamber at (from L to R) $R/R^{\ast }=1$ (onset of EC regime), 0.7, 0.4 and 0.1. (a,b) As expected, no statistically significant difference in ${\lambda }_E$ is observed for low volatility solvents (water, NMF); see Table 1. (c,d) In more volatile solvents (chloroform, DCM), pronounced differences in the thinning profiles and ${\lambda }_E$ result between trials taken inside versus outside the environmental control chamber

Figure 3

(a) Radius evolution, (b) extensional viscosities ${\eta }_E$, and (c) associated relaxation times ${\lambda }_E$ for semi-dilute PEO/chloroform solutions. Similar to PEO/water solutions, the EC regime lengthens (a) and ${\eta }_E$ increases (b) with increasing PEO concentration. (c) Relaxation times scale with concentration as ${\lambda }_E\propto c^{0.62}$. Error bars are the standard deviation across multiple trials; the shaded region reflects the 95% confidence interval around the fit to average values.

Figure 4

DoS images and schematic of proposed film formation for 3 mg/mL PEO/chloroform (no chamber). (a) Trial 1 exhibits subcritical film formation, where a connected film fails to span the entire bridge surface. (b) In trial 2, a critical film spans the entire surface, lengthening $t_b$ and ${\lambda }_E$ substantially. In both trials, the film has already formed by the time the drop contacts the substrate and begins to thin (frame 1). However, while trial 1 maintains axial symmetry, symmetry is broken in trial 2 and the surface film dominates thinning, explaining the large deviations in ${\lambda }_E$ and breakup time for PEO/chloroform trials performed outside of the chamber.