Automated Measurement of Spatially Resolved Hair-Hair Single Fiber Adhesion.

The adhesion force between individual human hair fibers in a crosshair geometry was measured by observing their natural bending and adhesive jumps out of contact, using optical video microscopy. The hair fibers' natural elastic responses, calibrated by measuring their natural resonant frequencies, were used to measure the forces. Using a custom-designed, automated apparatus to measure thousands of individual hair-hair contacts along millimeter length scales of hair, it was found that a broad, yet characteristic, spatially variant distribution in adhesion force is measured on the 1 to 1000 nN scale for both clean and conditioner-treated hair fibers. Comparison between the measured adhesion forces and adhesion forces modeled from the hairs' surface topography (measured using confocal laser profilometry) shows they have a good order-of-magnitude agreement and have similar breadth and shape. The agreement between the measurements and the model suggests, perhaps unsurprisingly, that hair-hair adhesion is governed, to a first approximation, by the unique surface structure of the hairs' cuticles and, therefore, the large distribution in local mean curvature at the various individual contact points along the hairs' lengths. We posit that haircare products could best control the surface properties (or at least the adhesive properties) between hairs by directly modifying the hair surface microstructure

Time-Dependent Physicochemical Changes of Carbonate Surfaces from SmartWater (Diluted Seawater) Flooding Processes for Improved Oil Recovery.

Over the past few decades, field- and laboratory-scale studies have shown enhancements in oil recovery when reservoirs, which contain high-salinity formation water (FW), are waterflooded with modified-salinity salt water (widely referred to as the low-salinity, dilution, or SmartWater effect for improved oil recovery). In this study, we investigated the time dependence of the physicochemical processes that occur during diluted seawater (i.e., SmartWater) waterflooding processes of specific relevance to carbonate oil reservoirs. We measured the changes to oil/water/rock wettability, surface roughness, and surface chemical composition during SmartWater flooding using 10-fold-diluted seawater under mimicked oil reservoir conditions with calcite and carbonate reservoir rocks. Distinct effects due to SmartWater flooding were observed and found to occur on two different timescales: (1) a rapid (<15 min) increase in the colloidal electrostatic double-layer repulsion between the rock and oil across the SmartWater, leading to a decreased oil/water/rock adhesion energy and thus increased water wetness and (2) slower (>12 h to complete) physicochemical changes of the calcite and carbonate reservoir rock surfaces, including surface roughening via the dissolution of rock and the reprecipitation of dissolved carbonate species after exchanging key ions (Ca2+, Mg2+, CO32-, and SO42- in carbonates) with those in the flooding SmartWater. Our experiments using crude oil from a carbonate reservoir reveal that these reservoir rock surfaces are covered with organic-ionic preadsorbed films (ad-layers), which the SmartWater removes (detaches) as flakes. Removal of the organic-ionic ad-layers by SmartWater flooding enhances oil release from the surfaces, which was found to be critical to increasing the water wetness and significantly improving oil removal from carbonates. Additionally, the increase in water wetness is further enhanced by roughening of the rock surfaces, which decreases the effective contact (interaction) area between the oil and rock interfaces. Furthermore, we found that the rate of these slower physicochemical changes to the carbonate rock surfaces increases with increasing temperature (at least up to an experimental temperature of 75 °C). Our results suggest that the effectiveness of improved oil recovery from SmartWater flooding depends strongly on the formation of the organic-ionic ad-layers. In oil reservoirs where the ad-layer is fully developed and robust, injecting SmartWater would lead to significant removal of the ad-layer and improved oil recovery

Characterizing the Dynamic Interactions of Biological and Biologically-Inspired Surfaces and Interfaces

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

Characterizing the Dynamic Interactions of Biological and Biologically-Inspired Surfaces and Interfaces

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

Effects of molecular weight of grafted hyaluronic acid on wear initiation

Hyaluronic acid (HA) of different molecular weights (M-w) was grafted onto mica surfaces to study the effects of M-w on the conformation and wear protection properties of a grafted HA (gHA) layer in lubricin (LUB) and bovine synovial fluid (BSF) using a surface forces apparatus. The M-w of gHA had significant effects on the wear pressure (P-w), at which point the wear initiates. Increasing the gHA M-w from 51 to 2590 kDa increased P-w from 4 to 8 MPa in LUB and from 15 to 31 MPa in BSF. The 2590 kDa gHA in BSF had the best wear protection (P-w similar to 31 MPa), even though it exhibited the highest friction coefficient (mu similar to 0.35), indicating that a low mu does not necessarily result in good wear protection, as is often assumed. The normal force profile indicated that BSF confines the gHA structure, making it polymer brush-like, commonly considered as an excellent structure for boundary lubrication.close4

Real-time intermembrane force measurements and imaging of lipid domain morphology during hemifusion.

Membrane fusion is the core process in membrane trafficking and is essential for cellular transport of proteins and other biomacromolecules. During protein-mediated membrane fusion, membrane proteins are often excluded from the membrane-membrane contact, indicating that local structural transformations in lipid domains play a major role. However, the rearrangements of lipid domains during fusion have not been thoroughly examined. Here using a newly developed Fluorescence Surface Forces Apparatus (FL-SFA), migration of liquid-disordered clusters and depletion of liquid-ordered domains at the membrane-membrane contact are imaged in real time during hemifusion of model lipid membranes, together with simultaneous force-distance and lipid membrane thickness measurements. The load and contact time-dependent hemifusion results show that the domain rearrangements decrease the energy barrier to fusion, illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems

Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces

Geckos have developed foot pads that allow them to maintain their unique climbing ability despite vast differences of surfaces and environments, from dry desert to humid rainforest. Likewise, successful gecko-inspired mimics should exhibit adhesive and frictional performance across a similarly diverse range of climates. In this work, we focus on the effect of relative humidity (RH) on the “frictional-adhesion” behavior of gecko-inspired adhesive pads. A surface forces apparatus was used to quantitatively measure adhesion and friction forces of a microfibrillar cross-linked polydimethylsiloxane surface against a smooth hemispherical glass disk at varying relative humidity, from 0 to 100% (including fully submerged under water). Geometrically anisotropic tilted half-cylinder microfibers yield a “grip state” (high adhesion and friction forces after shearing along the tilt of the fibers, <i>F</i>_ad⁺ and <i>F</i>_∥⁺) and a “release state” (low adhesion and friction after shearing against the tilt of the fibers, <i>F</i>_ad^– and <i>F</i>_∥^–). By appropriate control of the loading path, this allows for transition between strong attachment and easy detachment. Changing the preload and shear direction gives rise to differences in the effective contact area at each fiber and the microscale and nanoscale structure of the contact while changing the relative humidity results in differences in the relative contributions of van der Waals and capillary forces. In combination, both effects lead to interesting trends in the adhesion and friction forces. At up to 75% RH, the grip state adhesion force remains constant and the ratio of grip to release adhesion force does not drop below 4.0. In addition, the friction forces <i>F</i>_∥⁺ and <i>F</i>_∥^– and the release state adhesion force <i>F</i>_ad^– exhibit a maximum at intermediate relative humidity between 40% and 75%

Real-time intermembrane force measurements and imaging of lipid domain morphology during hemifusion.

Membrane fusion is the core process in membrane trafficking and is essential for cellular transport of proteins and other biomacromolecules. During protein-mediated membrane fusion, membrane proteins are often excluded from the membrane-membrane contact, indicating that local structural transformations in lipid domains play a major role. However, the rearrangements of lipid domains during fusion have not been thoroughly examined. Here using a newly developed Fluorescence Surface Forces Apparatus (FL-SFA), migration of liquid-disordered clusters and depletion of liquid-ordered domains at the membrane-membrane contact are imaged in real time during hemifusion of model lipid membranes, together with simultaneous force-distance and lipid membrane thickness measurements. The load and contact time-dependent hemifusion results show that the domain rearrangements decrease the energy barrier to fusion, illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems

Stick-slip friction of gecko-mimetic flaps on smooth and rough surfaces.

The discovery and understanding of gecko 'frictional-adhesion' adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick-slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick-slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick-slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems

Stick-slip friction of gecko-mimetic flaps on smooth and rough surfaces.

The discovery and understanding of gecko 'frictional-adhesion' adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick-slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick-slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick-slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems