Dewetting of Polymer Films Controlled by Protein Adsorption

Polarity-Induced Reactive Wetting: Spreading and Retracting Sessile Water Drops

Wetting is typically defined by the relative liquid to solid surface tension/energy, which are composed of polar and nonpolar subcontributions. Current studies often assume that they remain invariant, that is, surfaces are wetting-inert. Complex wetting scenarios, such as adaptive or reactive wetting processes, may involve time-dependent variations in interfacial energies. To maximize differences in energetic states, we employ low-energy perfluoroalkyls integrated with high-energy silica-based polar moieties grown on low-energy polydimethylsiloxane. To this end, we tune the hydrophilic-like wettability on these perfluoroalkyl-silica-polydimethylsiloxane surfaces. Drop contact behaviors range from invariantly hydrophobic at ca. 110° to rapidly spreading at ca. 0° within 5 s. Unintuitively, these vapor-grown surfaces transit toward greater hydrophilicity with increasing perfluoroalkyl deposition. Notably, this occurs as sequential silica-and-perfluoroalkyl deposition also leaves behind embedded polar moieties. We highlight how surfaces having such chemical heterogeneity are inherently wetting-reactive. By creating an abrupt wetting transition composed of reactive and inert domains, we introduce spatial dependency. Drops contacting the transition spread before retracting, occurring over the time scale of a few seconds. This phenomenon contradicts current understanding, exhibiting a uniquely (1) decreasing advancing contact angle and (2) increasing receding contact angle. To explain the behavior, we model such time- and space- dependent reactive wetting using first order kinetics. In doing so, we explore how reactive and recovery mechanisms govern the characteristic time scales of spreading and retracting sessile drops.

Mattel's ©Barbie: Preventing Plasticizers Leakage in PVC Artworks and Design Objects through Film-Forming Solutions

The main conservation problem of p-PVC artworks is phthalate-based plasticizer migration. Phthalate migration from the bulk to the surface of the materials leads to the formation of a glossy and oily film on the outer layers, ultimately reducing the flexibility of the material. This study aimed to develop a removable coating for the preservation of contemporary artworks and design objects made of plasticized polyvinyl chloride (p-PVC). Several coatings incorporating chitosan, collagen, and cellulose ethers were assessed as potential barriers to inhibiting plasticizer migration. Analytical techniques including optical microscopy (OM), ultraviolet/visible/near-infrared spectroscopy (UV/Vis/NIR), Fourier transform infrared spectroscopy with attenuated total reflection (FTIR-ATR), and scanning electron microscopy (SEM) were utilized to evaluate the optical and chemical stability of selected coating formulations applied to laboratory p-PVC sheet specimens. Subsequently, formulations were tested on a real tangible example of a design object, ©Barbie doll, characterized by the prevalent issue of plasticizer migration. Furthermore, the results obtained with the tested formulations were evaluated by a group of conservators using a tailored survey. Finally, a suitable coating formulation capable of safeguarding plastic substrates was suggested.

Roles of aqueous nonsolvents influencing the dynamic stability of poly-(n-butyl methacrylate) thin films at biologically relevant temperatures

Poly-(n-butyl methacrylate) (PnBMA) is an important polymer in biomedical applications. Here we study the stability of PnBMA thin films prepared on top of slippery silicon substrates and exposed to nonsolvent aqueous incubation media like water and phosphate-buffered saline (PBS) at temperatures relevant to biological applications (37 °C, 25 °C and 4 °C). Dewetting hole growth experiments allowed us to probe the instability in PnBMA films upon incubation followed by thermal annealing. From the early stage of dewetting hole growth dynamics, we inferred that the stability of the thin PnBMA films decreases as a function of the duration and temperature of incubation, even though the films were found not to readily dewet at room temperature after incubation. It is also observed that water incubation makes films more unstable than incubation in PBS. We explained our observations as a combined effect of (i) an increase in surface energy of the PnBMA film due to incubation, (ii) an increased destabilizing effect due to the dominant polar interactions between the incubation medium and the PnBMA film and (iii) the plasticization effect of PnBMA films by the incubation media. Plasticization resulted in a decrease in the modulus of PnBMA thin films as a function of incubation time. The viscosity of PnBMA films upon incubation was found to be coupled to the decreasing modulus. Thus we infer that incubation in common aqueous nonsolvents can detrimentally affect the stability of polymers limiting their specific usages through a complex interplay of multiple molecular level phenomena.

Assessing Polymer-Surface Adhesion with a Polymer Collection

Polymer modification plays an important role in the construction of devices, but the lack of fundamental understanding on polymer-surface adhesion limits the development of miniaturized devices. In this work, a thermoplastic polymer collection was established using the combinatorial laser-induced forward transfer technique as a research platform, to assess the adhesion of polymers to substrates of different wettability. Furthermore, it also revealed the influence of adhesion on dewetting phenomena during the laser transfer and relaxation process, resulting in polymer spots of various morphologies. This gives a general insight into polymer-surface adhesion and connects it with the generation of defined polymer microstructures, which can be a valuable reference for the rational use of polymers.

Patterning of a Drying Emulsion Film

Stabilizing layers of colloidal dispersions or emulsions to obtain homogeneous films is a real challenge. We describe here a new kind of instability in drying films of emulsions: during evaporation of the internal phase, cracks appear between the droplets that create aggregates according to a regular pattern. We show that this pattern only appears if the emulsion is adhesive, i.e., if droplets stick together. The pattern exhibits a characteristic length which depends on the adhesion strength and film thickness. These experimental results support a model where this instability is due to the gel structure and elastic properties of adhesive emulsions. Understanding this phenomenon will allow us to get a homogeneous film or to control it to get structured materials.