Adam J. Nolte

Surface Wrinkling: A Versatile Platform for Measuring Thin‐Film Properties

Surface instabilities in soft matter have been the subject of increasingly innovative research aimed at better understanding the physics of their formation and their utility in patterning, organizing, and measuring materials properties on the micro and nanoscale. The focus of this Review is on a type of instability pattern known as surface wrinkling, covering the general concepts of this phenomenon and several recent applications involving the measurement of thin-film properties. The ability of surface wrinkling to yield new insights into particularly challenging materials systems such as ultrathin films, polymer brushes, polyelectrolyte multilayer assemblies, ultrasoft materials, and nanoscale structured materials is highlighted. A perspective on the future directions of this maturing field, including the prospects for advanced thin-film metrology methods, facile surface patterning, and the control of topology-sensitive phenomena, such as wetting and adhesion, is also presented.

Avoiding Cracks in Nanoparticle Films

A new method utilizing subsequent depositions of thin crack-free nanoparticle layers is demonstrated to avoid the formation of cracks within silica nanoparticle films. Using this method, films can be assembled with thicknesses exceeding the critical cracking values. Explanation of this observed phenomenon is hypothesized to mainly arise from chemical bond formation between neighboring silica nanoparticles. Application of this method for fabricating crack-free functional structures is demonstrated by producing crack-free Bragg reflectors that exhibit structural color.

One-step index-tunable antireflection coatings from aggregated silica nanoparticles.

We demonstrate a method for producing thickness- and refractive index-tunable antireflection coatings utilizing a one-step spin coating procedure with silica nanoparticle solutions. Aging nanoparticle solutions under controlled pH and temperature induces aggregation, allowing precise control of the porosity and refractive index of the spin-processed coating. Coating thickness measurements as a function of solution aging time and temperature allow for determination of the activation energy of the reaction-limited aggregation process. We demonstrate optimization of the antireflection effect for a single-layer silica nanoparticle coating on glass, and suggest that the aggregation method may be generalized to various other nanoparticle-based assemblies.

In situ Adhesion Measurements Utilizing Layer-by-Layer Functionalized Surfaces

The adhesion between poly(dimethylsiloxane) (PDMS) hemispheres coated with layer-by-layer (LbL) assemblies of polyelectrolytes and rigid, planar substrates was investigated using Johnson, Kendall, and Roberts (JKR) contact mechanics. Measurements were performed against amine-functionalized glass slides both in air and in aqueous solutions of controlled pH. Despite the increased density of negatively charged carboxylate groups, LbL-functionalized PDMS exhibited lower adhesion because of the combined effects of increased surface roughness and the high Youngs modulus of the coating. Measurements of coated PDMS in aqueous solutions revealed tunable adhesion behavior dominated by pH-mediated changes in the mechanical properties of the coating. Smoothing the surface of the LbL coatings by aqueous salt annealing led to a significant increase in adhesion. Our results suggest that LbL assembly can be an effective means of surface functionalization for in situ adhesion measurements, but understanding and predicting the adhesion behavior requires comprehensive knowledge of the chemical, mechanical, and topological properties of the coating and how such properties change in response to the ambient environment.