Most fundamental theories on interfacial interactions consist of equilibrium phenomena, yet many processes and interactions in practice occur outside of these equilibria in a dynamic state. This thesis explores how biological and biologically-inspired surfaces and interfaces interact with each other under dynamic conditions during adhesion, friction, and lubrication. Using a surface forces apparatus, new methodologies and analyses have been designed to study such dynamic interactions. This thesis takes a tour from fully dry systems, to humidified, lubricated, and finally fully submerged systems underwater.

The chapters are divided into unique systems including gecko-inspired adhesives (Chapter 2), complex moisturizing fluids (skin creams) (Chapter 3), and lipid membranes (Chapter 4). Chapter 2 investigates the frictional adhesion characteristics of microfibrillar gecko-mimetic adhesives in diverse environments. The discovery of the underlying mechanisms for attaining grip against rough surfaces and in humid conditions has led to new design principles for future gecko-mimetic adhesives.

Chapter 3 explores how to test for and quantify high frequency dynamic friction force components in a complex lubricating film of skin cream. The instrumentation and analysis methods can be applied to any such high-speed friction experiment to uncover and unambiguously differentiate stick-slip and oscillatory friction behavior in a diverse range of systems.

Lastly, Chapter 4 discusses the discovery of dynamic lipid membrane domain rearrangements during hemifusion. New instrumentation (Fluorescence Surface Forces Apparatus – FL-SFA) was developed to simultaneously measure the interfacial forces between apposing membranes and visualize in situ morphological changes occurring at the interface. This study discovered lipid rearrangements that occur in cell-cell interactions including cellular transport. The thesis concludes with the future potential of the newly developed FL-SFA device and technique.

In aqueous solutions—such as physiological fluids, seawater, or detergent solutions—both adhesion and cohesion between surfaces are severely limited by high salt concentrations, oxidizing pH levels, and bound hydration layers at the solid-liquid interface. While problematic, these limitations do not entirely prohibit adhesion: certain synthetic polymers and biological molecules exhibit adaptive amphiphilic interactions that strongly bind to wet surfaces and lead to robust adhesion. Identifying the intra- and inter-molecular interactions between adhesive molecules and surfaces—including hydrogen bonding, electrostatic interactions, solvation interactions, polymer dynamics, and synergistic interactions—is imperative for the future design of high-performance wet adhesives and materials. In the following thesis, a surface forces apparatus (SFA) and other surface characterization techniques are used to study how unique molecules display concurrent adhesive mechanisms, adapt, self-assemble, and partition between chemically heterogeneous surfaces (either hydrophobic or hydrophilic) to achieve durable wet adhesion.

This thesis is divided into chapters on surfactant and polyelectrolyte self-assembly and adhesion (Chapter 2) and bio-inspired peptidyl adhesion (Chapters 3 and 4). Chapter 2 explores the behaviors of aliphatic surfactants and silicone polyelectrolytes as they self-assemble at hydrophobic interfaces and mediate strong adhesion to mineral surfaces. In this section, the molecular geometries of surfactants and polyelectrolytes are shown to control the self-assembled structures that form at aqueous surfaces, as well as the overall adhesion between surfaces in solution.

Chapters 3 and 4 investigate the molecular origins of peptide-based wet adhesion. Certain sessile marine organisms, particularly mussels, robustly attach to wet and chemically heterogeneous surfaces through protein adhesive plaques that contain high concentrations of the catecholic amino acid 3,4-dihydroxyphenylalanine (Dopa). In Chapter 3, SFA measurements and molecular dynamics simulations reveal how Dopa and other amino acids in mussel foot proteins (Mfps) or peptides partition between hydrophobic or hydrophilic organic surfaces, leading to an adaptive adhesion mechanism. Chapter 4 explores the synergistic interaction between catechol and cationic amino acids (such as lysine and arginine) in surface adhesion. Through SFA measurements with siderophores—bacterial iron-chelators that consist of paired catechol and cationic moieties—it is shown that adjacent catechol-cation placement provides a “1-2 punch”, whereby cationic amino acids evict hydrated salt ions from a mineral surface, allowing catechol binding to underlying oxides. Overall, these results provide a compelling rationale for the >20 mole% of cationic residues in Dopa-rich Mfps and establish a set of design parameters for future bio-inspired synthetic polymers.

Biological systems have developed elegant adaptations during its evolution to survive and perform its functions efficiently under specific environmental constrains with enormous physical demands. In this dissertation, I make an effort to understand tribological phenomena in biology and translate them into a synthetic system for engineering applications. I emphasize on adhesion, friction and lubrication in three different biologically inspired soft condensed matter as described below.

Dopa (3,4-dihydroxyphenylalanine), a post-translational modification from tyrosine (Tyr), features prominently in the mussel foot proteins (mfps), ranging from less than 5 mol % in mfp-4 to 30 mol % in mfp-5. The binding ability of the mfps to different substrates has been mostly attributed to the Dopa functionality in the protein and the role of the other peptide residues in the adhesive properties of the protein remains elusive. Here we have discovered that the adhesion between mfp-1 decapeptide films ([AKPSYPPTYK]2) and mica remained unchanged with or without the Dopa residue. This is a paradigm shift in our understanding of the molecular mechanisms underlying adhesive properties of the mfps and calls for further inquiry into the effects of peptide residues beyond Dopa chemistry. We also developed a systematic body of work linking the adhesive performance to lengths and architectures of peptides. Dopa in a peptide sequence does not necessarily lead to the formation of cross-links between peptide films through metal chelation, and the length of the peptide is a crucial parameter for enabling metal ion mediated bridging between surfaces. More recently, we have been working on designing and characterizing small molecules that mimic the properties of the adhesive mussel foot proteins. The wet adhesion and coacervation of an adhesive protein (mfp-5) was recapitulated in an order of magnitude smaller length scale which shows cohesive properties superior to the mfps. We believe that the resulting insights into the molecular structure-function relationships will enable rational design of synthetic bio-inspired adhesives that would enable de novo (suture less) sealants for injuries and surgeries and nano-scale-adhesive applications in the semiconductor industry.

Geckos can attach and detach their toes reversible in matters of milliseconds from most surfaces regardless of its roughness due to the hierarchical structure of their foot-pads. Micro-flaps mimicking the function of the micron sized setae on the gecko foot pad were fabricated and investigated for its adhesion and frictional properties in a modified surface forces apparatus (SFA). A Johnson-Kendall-Roberts (JKR) model with an effective stiffness and adhesion energy parameters quantitatively described the `contact mechanics' of the tilted micro-flaps against a smooth silica surface at the macro and micro-scales. Constant attachments and detachments occurred between the surfaces during shearing and were described by an Avalanche mechanism. These results demonstrate the significance of preload, shearing velocity, shearing distances, commensurability, and shearing direction of gecko-mimetic adhesives and provide a simple model for analyzing and/or designing such systems.

Biolubrication systems show ultralow friction coefficients, remarkable wear resistance properties and are far superior to any artificial system designed to date. In this work, the role of proteins (e.g., Lubricin, Lub) and polysaccharides (e.g., Hyaluronic acid, HA) found in articular joints, and mfp-1 inspired coacervates were investigated to determine the lubrication and wear protection mechanisms conferred by the naturally occurring polymers to a mica surface. We find that Lub penetrates into a chemically bound HA on mica to form a visco-elastic gel that reduces the coefficient of friction as well as boosts the strength of the surface against abrasive wear, however, physically adsorbed HA-Lub complex were poor at conferring wear protection to mica even though it showed low friction coefficients. Similarly, coacervated mfp-1/HA rescues mica from shear induced damage only when the protein is modified with Dopa, which is responsible for attaching the coacervate to the surface. Absence of Dopa resulted in severe abrasive wear to the surfaces even under low loads (< 10 mN) during shearing. These results show that strong anchoring of polymers is crucial to protect surfaces from shear induced damage. We also demonstrate that friction coefficient is not correlated to wear.

Electrostatic interactions determine the key properties of several materials that promise to enable revolutionary nanotechnologies. For example, the power delivered by ionic liquid-based supercapacitors, and the curing time of bio-inspired surgical adhesives, both hinge on the collective behavior of large ensembles of ions in confined interfaces. However, a comprehensive framework for understanding and influencing electrostatic interactions in highly concentrated electrolytes remains elusive. We address this knowledge gap by using nanoscale force-distance measurements to investigate and tune electrostatic interactions in interfaces containing high ionic densities, with an aim of providing quantitative molecular foundations to guide nanomaterials design.

In this dissertation, we first review our extension of the Surface Forces Apparatus technique (SFA) to enable force-distance measurements with in situ electrochemical control (the EC SFA). Next, we discuss how EC SFA experiments were combined with temperature-controlled SFA measurements to demonstrate that the electrostatic screening properties of ionic liquids can be rationalized using mean-field theories by focusing on local excess charge densities, as opposed to explicitly accounting for the distribution of every ion. Finally, we illustrate the pivotal role of electrostatic interactions in governing the adhesive performance of mussel-mimetic peptides and conclude with an analysis of how natural proteins provide molecular themes that may guide the design of biomedical adhesives.

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.

Many important material properties are determined by the surface layer (0.5-100 nm); these include optoelectronic properties like conductivity, absorptivity, and reflectance as well as physiochemical properties such as molecular adsorption, hydrophobicity, and surface diffusivity. Furthermore, the crystallization or assembly processes of technologically-important nanomaterials are mediated by surface interactions and influence the surface compositions and properties of the resultant material, including zeolites, colloidal semiconductors, carbon-based electrocatalysts, and mesoporous inorganic oxides. Catalytic reaction properties of diverse porous heterogeneous materials are determined by molecular interactions of adsorbates, reactants, or product species at pore or exterior surface sites.

Despite the broad importance of surface interactions in determining material properties, fundamental questions remain regarding the physiochemical interactions at surfaces that determine crystallization, adsorption, optoelectronic, and/or reaction properties. This is because such properties often depend on dilute surface moieties, defect species, and/or molecular adsorbates that occupy distributions that are partially- or non-ordered and are therefore challenging or impossible to characterize by conventional scattering techniques. Developing atomic-level insights into the types, interactions, and distributions of such dilute non-ordered species is crucial to elucidate the molecular-level origins of the properties and synthesis pathways of materials such as zeolites, semiconductor nanoparticles, and mesoporous electrocatalysts. By understanding the crystallization, synthesis, and assembly processes of these materials, as well as the resulting structures and active species, the resulting insights can be applied to develop new synthetic or post-synthetic treatments to generate more effective, stable, and/or active materials with desirable properties.

The objective of this dissertation is to measure, understand, and correlate the atomic-scale compositions, structures, and properties of heterogenous materials with diverse applications, including heterogeneous catalysis and solid-state lighting. Recently-developed solid-state nuclear magnetic resonance (NMR) techniques with complementary X-ray diffraction and electron microscopy analyses are applied to elucidate the structures and compositions of dilute surface, defect, or heteroatom species, which are correlated to the macroscopic properties of interest. These techniques are applied to analyze diverse heterogeneous inorganic nanomaterials, including aluminosilicate zeolites, cementitious solids, precious-metal-free electrocatalysts, and nanocrystalline semiconductors. Though the material systems vary in composition and application, in each case the important optical, electronic, and/or catalytic properties arise from dilute partially- or non-ordered defect or heteroatom species in a semi-crystalline lattice. The overall unifying themes are: (1) analysis of order and disorder in semi-crystalline inorganic solids using state-of-the-art diffraction and spectroscopic characterization techniques; (2) determining the distributions and structures of non-stoichiometric species, particularly at surfaces and interfaces; and (3) correlating atomic-level structures and compositions with macroscopic material properties. The insights provided are of broad importance and relevance to diverse material systems of technological interest for sustainable energy storage, conversion, and utilization.