Chelsea S. Davis

Mechanics of wrinkled surface adhesion

Surface buckling instabilities, particularly wrinkles, are spontaneously occurring surface patterns that can cover large areas and have the potential to modify the adhesion of surfaces in a systematic manner; however, the impact of these wrinkled features is not understood quantitatively. We utilize a novel fabrication process to form aligned wrinkles from polystyrene and polydimethylsiloxane and quantify their adhesion using contact adhesion tests. Wrinkle amplitudes range from 0.3 μm to 11.4 μm and wavelengths range from 6.2 μm to 74.0 μm, and these two parameters are tuned independently. The maximum separation force of a flat cylindrical probe from a wrinkled surface depends nonlinearly on the wrinkle geometry, as described by both amplitude and wavelength. Additionally, results are presented for a set of adhesion experiments conducted on single, macroscopic cylinders using small circular flat probes to mimic the contact of individual wrinkles. A simple scaling is presented that incorporates geometric parameters, testing geometry and materials properties to predict the separation force. This relationship is shown to be in good agreement with the experimental data.

Contact-line mechanics for pattern control

Wrinkled surfaces are ubiquitous in Nature and can be used in a large range of applications such as improved adhesives, microfluidic patterns, or as metrology instruments. Despite wide-ranging applications, existing methods do not permit local pattern control since all existing methods impose extensive compressive strains. In this article, we describe a new process that exploits the local deformation of a soft substrate as it stretches to form an adhesive interface with a thin polymer film. The wrinkle pattern is effectively a measurement of the strain-field created during the adhesion process, which shows a strong dependence on the speed of attachment. We develop simple scaling arguments to describe this velocity dependence and a critical velocity above which wrinkles do not form. Notably, our approach allows us to define the surface pattern “wrinkle-by-wrinkle”, thus permitting the creation of single wrinkles. Intricate patterns on laterally extensive length scales can also be produced by exploiting the shape of the contact line between the film and the substrate. This level of control—the placement of single features of prescribed trajectory—which is not present in any other method of thin film wrinkling, is absolutely necessary for any realistic, scalable application.

Debonding mechanisms of soft materials at short contact times.

A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked poly(dimethylsiloxane) (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined Johnson-Kendall-Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMS-glass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 to 10,000 ms. This influence of dwell time is particularly pronounced for polymer-polymer (SBR-SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported.

A Study of the Adhesive Foot of the Gecko: Translation of a Publication by Franz Weitlaner

Although it seems that gecko adhesion research is a relatively young branch of science, this recently rediscovered work presents some very old studies with quite remarkable findings. The publication of Dr. F. Weitlaner from 1902 is very impressive, as it covers many recently published topics and – even more impressively – often comes to the same conclusions and provides similar results compared with current publications. Weitlaner published his paper in German which was – at that time – very common in science. This makes it almost impossible for todays international community of bioinspired adhesion researchers to enjoy and appreciate this early gem of scientific work. Thus, we have decided to translate his paper in the hope that it finds the attention it deserves and that it inspires us to stay curious and pursue answers to the questions which have been asked for over a century.

Effect of fiber gripping method on the single fiber tensile test: II. Comparison of fiber gripping materials and loading rates

Single poly(p-phenylene terephthalamide) (PPTA) fiber tensile tests were carried out under quasi-static and high strain rate loading conditions using poly(methyl methacrylate) and rubber grips to investigate effects of grip materials and loading rates on fiber tensile properties. Differences in ultimate tensile strengths, failure strains, and moduli of PPTA fibers obtained by two different grip materials were insignificant. On the other hand, the fiber tensile properties showed significantly rate-dependent behaviors, which were graphically confirmed by kernel density plots as a non-parametric statistical analysis. Strength models considering three aspects (stochastic, fracture mechanics, and polymer chain domain behaviors) were also shown to link the loading rate effect in relation to fracture mechanisms.

Epoxy Composites Using Wood Pulp Components as Fillers

The components of wood, especially lignin and cellulose, have great potential for improving the properties of polymer composites. In this chapter, we discuss some of the latest developments from our lab on incorporating wood based materials into epoxy composites. Lignosulfonate was used as a flame retardant and cellulose nanocrystals were used as reinforcing materials. Lignosulfonate will disperse well in epoxy, but phase separates during curing. An epoxidation reaction was developed to immobilize the lignosulfonate during curing. The lignosulfonate – epoxy composites are characterized using microcombustion and cone calorimetry tests. Cellulose also has poor interfacial adhesion to hydrophobic polymer matrixes. Cellulose fibers and nanocrystals aggregate when placed in epoxy resin, resulting in very poor dispersion. The cellulose nanocrystal surface was modified with phenyl containing materials to disrupt cellulose interchain hydrogen bonding and improve dispersion in the epoxy resin. The cellulose nanocrystal – epoxy composites were characterized using tensile tests and microscopic techniques.