UC Santa Barbara

Interfacial phenomena and materials properties are not only influenced by the chemical makeup, and therefore the highly localized intermolecular forces, between surface and bulk molecules, but also can heavily depend on geometry, self-assembly, dynamic/transient interactions, and the development of specific micro- and nano-structure. This general principle can be applied to a variety of systems including inorganic materials, polymeric materials, complex fluids and colloidal systems, and biological and bio-inspired materials. This dissertation explores how micro-/nano-structure and dynamic phenomena influence the macroscopic properties and performance of two broad classes of materials: (i) mussel-mimetic polymer materials and adhesives and (ii) consumer skincare and hair care products.

In Part 1 (Chapters 2 and 3) of this dissertation, two different materials systems inspired by Mytilus mussels have been synthesized. Chapter 2 presents nanoscale adhesion and cohesion force measurements of a mussel-mimetic adhesive peptoid between mica surfaces to further elucidate the mechanisms mussels employ to achieve robust wet adhesion. Comparing these results to those measured using peptide molecules of identical sequence, we determine that nano-scale aggregation and the formation of transient secondary structure, driven by backbone hydrogen bonding, exists only in the peptide molecules, causing differences in the nano-structure and hydration of solution-deposited films in each system, and therefore influencing the intermolecular interactions available between opposing surfaces. Chapter 3 focuses on translating strong but reversible iron-catechol coordination chemistry found in the mussel’s byssal threads to dry epoxy networks in order to achieve enhanced mechanical strength and toughness through reversible energy dissipation. We find that there exists a critical iron-catechol complex concentration above which the materials develop an ionomeric nano-structure, which provides an additional mode of stress translation, thereby further increasing the material’s mechanical properties.

Part 2 (Chapters 4 and 5) shifts focus from mussel-mimetic materials to measuring the friction and adhesion of consumer skincare and hair care products. Chapter 4 explores the friction dynamics of model skin creams using a high-speed friction attachment to the surfaces forces apparatus. We demonstrate that analysis of the time-varying frequency components of friction signals can unambiguously differentiate between different modes of sliding, giving insights into how surface and fluid properties impact tribological properties. Finally, Chapter 5 details the development of a custom instrument designed to measure the exceedingly weak adhesion forces between human hair fibers both clean and coated with consumer hair care products. Calibrating the native elasticity of the hair fibers, we can use their own mechanical properties to measure forces as low as ~1 nN using a simple optical microscope. We find that the micro-/nano-structure on the hair surface causes broad, spatially-variant distributions in the adhesion force between fibers and necessitates large numbers of replicate experiments, which have been automated. A model which uses topographical measurements of the hairs’ surfaces has also been developed which can predict the magnitude, breadth, and shape of the complex, multimodal adhesion force distributions from the differential surface geometry.