Local, Real-Time Measurement of Drying Films of Aqueous Polymer Solutions Using Active Microrheology

Abstract

Local, real-time measurement of drying films of aqueous polymer solutions using active microrheology ABSTRACT: Oscillatory microdisk rheometry was applied to evaluate the evolution of the viscoelastic properties at the surface of a film of an aqueous solution of poly(vinyl alcohol) (PVA) during drying. The drying rate was measured concurrently, based upon measurements of the variation of film thickness. A fully hydrolyzed PVA solution shows a constant drying rate, while a less hydrolyzed PVA solution exhibits a decreased drying rate in the latter part of the drying process, which occurred at the same time as an increase of the elastic modulus. We suggest that this difference in behavior is a consequence of the fact that both the configuration of the PVA molecule and the strength of interaction with water depend on the degree to which the PVA is hydrolyzed. The polymer concentration at the film surface can be estimated from the measured viscosity at the surface for the fully hydrolyzed PVA solution, and this result then can be compared with two theoretical calculations: one in which the polymer concentration is assumed to remain uniform throughout the film, and the other in which the polymer concentration distribution is determined via a one-dimensional diffusion model. This comparison suggests that the polymer is first concentrated locally near the surface but later in the drying process the distribution of polymer becomes increasingly uniform, possibly due to a spontaneously generated convective flow inside the film.

Linear and nonlinear microrheometry of small samples and interfaces using microfabricated probes

“…We developed a novel microrheometer to actively measure rheological properties of different materials. This microrheometer has been used in a series of recent articles to study interfacial rheology [52][53][54][55][56]58,59]. Here, we extensively described the experimental apparatus and discussed data analysis so that interested readers can duplicate and use the microrheometer for their own purposes.…”

“…Having described the operation, calibration, and validation of the technique, we now focus on its capabilities with demonstratory measurements on various material systems, each of which highlights a distinct qualitative capability. Previously, we have used microbuttons to measure the linear and nonlinear surface rheological properties of insoluble phospholipid monolayers [53][54][55][56], small-molecule soluble surfactants [59], the evolving rheology of drying suspensions [58], and the nonlinear rheology of yielding colloidal monolayers [57]. We demonstrate that microbuttons are capable of measuring broad types of surface rheology, including linear surface viscosity of Newtonian-like insoluble monolayers, nonlinear surface rheology of surface-yielding materials and aging of surfaces.…”

“…The frequency range accessible to microbuttons is, of course, far smaller than for passive microrheology, and other limitations hold. We anticipate that such capabilities will be useful for measurements of small and precious samples, as well as evolving materials [58]. To demonstrate these capabilities, we present several proof-of-principle measurements of bulk material rheology: Simple Newtonian liquids, viscoelastic solutions, and measurements with microliter-scale sample volumes.…”

“…Thus far, we have used microbutton probes to measure the interfacial rheology of phospholipid monolayers [53][54][55][56], colloidal monolayers [52,57], drying suspensions [58], and soluble surfactants [59]. In this article, we aim to describe in detail its design, operation, calibration, and capabilities.…”

Viscous flow separation caused by the Marangoni effect in competition with capillary flow

“…1(b)), the inertial forces can be neglected compared to the viscous forces. Polymer solutions such as PVA/water reveal viscoelastic properties at high concentrations; 14 however, considering the low shear rates (γ < 0.0011/s) in this flow problem and the typical relaxation time λ < 1s of polymer solutions 15 results in Weissenberg numbers (Wi = λγ) much smaller than unity, and as a result, viscoelastic effects can be neglected. The term including viscosity derivative ∂µ ∂z ∂u ∂z in Eq.…”