Gang Pu

Characterization of Dynamic Stick-and-Break Wetting Behavior for Various Liquids on the Surface of a Highly Viscoelastic Polymer

A thermally stripped acrylic polymer was wet with a series of liquids possessing a broad range of properties. Previously, novel wetting behavior by water was reported for the polymer, which included the formation of a wetting ridge structure substantially larger than those reported elsewhere and the complete halting of the three-phase line. This allows metastable angles ranging from 0 degrees to greater than 150 degrees to be achieved through changes in the sessile drop volume. Greater advancing angles are prevented by the collapse of the drop, producing what has been described as stick-and-break propagation. In Wilhelmy plate experiments for metal plates coated with the polymer, this mechanism produces a quasi-periodic pattern of lines composed of ridge structures. Similar behavior was observed for all liquids tested. Differences were observed in the maximum force measured with a tensiometer (pinning force) and the average distance between ridges for the formed pattern (pinning distance). These quantities are shown to be related to the height of the ridge structures. The kinematic viscosity of the liquids appears to be an important variable for the wetting process. A comparison of pinning quantities at various rates with the master curve of the polymer indicate that its viscoelastic properties govern, to a great extent, the observed rate dependencies; i.e., higher rates produce greater elastic behavior and smaller ridge heights. Also important is the polymers tendency for creep deformation. The ridge apex is shown to be displaced a significant distance through ridge deformation, which modifies its symmetry.

Water Evaporation on Highly Viscoelastic Polymer Surfaces

Results are reported for a study on the evaporation of water droplets from a highly viscoelastic acrylic polymer surface. These are contrasted with those collected for the same measurements carried out on polydimethylsiloxane (PDMS). For PDMS, the evaporation process involves the expected multistep process including constant drop area, constant contact angle, and finally a combination of these steps until the liquid is gone. In contrast, water evaporation from the acrylic polymer shows a constant drop area mode throughout. Furthermore, during the evaporation process, the drop area actually expands on the acrylic polymer. The single mode evaporation process is consistent with formation of wetting structures, which cannot be propagated by the capillary forces. Expansion of the drop area is attributed to the influence of the drop capillary pressure. Furthermore, the rate of drop area expansion is shown to be dependent on the thickness of the polymer film.

Repositionable pressure-sensitive adhesive possessing thermal-stimuli switchable transparency

Described is the formulation for a repositionable pressure-sensitive adhesive (PSA) capable of reversibly switching between opaque and transparent via a thermal trigger. The smart adhesive is generated by casting films from a solution of polydimethylsiloxane (PDMS), its organometallic crosslinker and a paraffin wax/dodecane solution on poly(ethylene terephthalate) (PET) sheets. Once cured, the PDMS/paraffin wax composite films demonstrate characteristics consistent with commercial repositionable PSAs. Properties were found to be dependent on the paraffin wax content and degree of crosslinking in the PDMS. Performance of the developed adhesive films is highly stable remaining unchanged subsequent to extended use and repeated repositioning. The appearance of the films can be switched between opaque at room temperature to transparent for temperatures above 53 °C. The change in optical properties is nearly instantaneous and there appears to be no limitation on how many times the cycle can be repeated. The mechanism involved is unique involving the melting of the paraffin wax, which appears to allow for the formation of a more homogeneous composite structure. This adhesive formulation may be well suited for use as a low-cost, convenient smart window coating.

Drop Behavior on a Thermally-Stripped Acrylic Polymer: Influence of Surface Tension Induced Wetting Ridge Formation on Retention and Running

Results are reviewed from a study on retention and running of water and other liquids on tilted, polymer coated surfaces. The polymer is a thermally-stripped, solvent-borne acrylic composed primarily of the monomer 2-ethylhexyl acrylate, providing a soft and viscoelastic substrate absent of contaminants. It is shown that drop retention does not obey standard models, which assume dominance of capillary forces in offsetting drop weight for tilted plates. For these surfaces, maximum volumes correlate with capillary lengths, and distinct deformations, which vary in magnitude depending on location, are apparent over the entire drop perimeter. Deformation images indicate that running, which in real time appears to be continuous motion, actually proceeds through a series of steps beginning with the failure of the front edge wetting line. This produces a relatively large translation of the drops front edge down the plate surface stretching the drop. This is followed by multiple failures at the rear edge producing a series of small translations, contracting the drop volume to a more spherical-like geometry. Repetition of this mechanism results in the appearance of propagation similar to that employed by an inchworm. The proposed mechanism is consistent with images of drop movement and deformations induced on polymer surfaces, which are apparent subsequent to the running process.

Variety of wetting line propagations demonstrated on viscoelastic substrate

Propagation of wetting lines for various interfaces formed between oil, water, and air on a highly viscoelastic, polymeric surface were investigated. The wetting of the acrylic polymer shows complex as well as diverse patterns. Stick and break motions of the three-phase line, characteristic of its halting due to ridge formation, appeared in either advancing or receding motions, in both directions or not at all. Such experiments demonstrate the various wetting behaviors possible on a soft, viscoelastic surface. A comprehensive explanation based on the vertical capillary force component is provided, which is consistent with these as well as previous observations.

Stabilizing contact angle hysteresis of paraffin wax surfaces with nanoclay

The addition of montmorillonite clay modified with an alkylammonium salt surfactant (i.e., organoclay) to paraffin wax is found to reduce the decay in wetting properties associated with its heating in the melt. It was previously shown that holding wax in its molten form prior to characterization reduces crystallinity when the solid forms. This results in the development of microscale amorphous regions at wax surfaces, which appear to be more polar given the abundance of methylene linkages versus methyl groups. These regions are believed to impact the receding angles for more polar liquids almost exclusively. It is known that the introduction and exfoliation of a small amount of the organoclay greatly enhances the stiffness, strength, and toughness of paraffin wax. Here, it is shown that the organoclay also promotes the formation of coatings possessing fewer thermal cracks and helps maintain higher crystallinity levels. Fresh wax surfaces containing the clay are slightly rougher than those without, which produces a slight increase in hysteresis. However, the significant drops in receding angles found for paraffin wax samples cast from the melt subsequent to heating are absent.