Patricia McGuiggan

Liquid to solidlike transitions of molecularly thin films under shear

We have measured the shear forces between two molecularly smooth solid surfaces separated by thin films of various organic liquids. The aim was to investigate the nature of the transitions from continuum to molecular behavior in very thin films. For films whose thickness exceeds ten molecular diameters both their static and dynamic behavior can usually be described in terms of their bulk properties, but for thinner films their behavior becomes progressively more solidlike and can no longer be described, even qualitatively, in terms of bulk/continuum properties such as viscosity. The solidlike state is characterized by the ordering of the liquid molecules into discrete layers. The molecular ordering is further modified by shear, which imposes a preferred orientation. All solidlike films exhibit a yield point or critical shear stress, beyond which they behave like liquid crystals or ductile solids undergoing plastic deformation. Our results on five liquids of different molecular geometry reveal some very co...

Adhesion and friction in gecko toe attachment and detachment

Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of ≈20 ms. The rapid switching between gecko foot attachment and detachment is analyzed theoretically based on a tape model that incorporates the adhesion and friction forces originating from the van der Waals forces between the submicron-sized spatulae and the substrate, which are controlled by the (macroscopic) actions of the gecko toes. The pulling force of a spatula along its shaft with an angle θ between 0 and 90° to the substrate, has a “normal adhesion force” contribution, produced at the spatula-substrate bifurcation zone, and a “lateral friction force” contribution from the part of spatula still in contact with the substrate. High net friction and adhesion forces on the whole gecko are obtained by rolling down and gripping the toes inward to realize small pulling angles θ between the large number of spatulae in contact with the substrate. To detach, the high adhesion/friction is rapidly reduced to a very low value by rolling the toes upward and backward, which, mediated by the lever function of the setal shaft, peels the spatulae off perpendicularly from the substrates. By these mechanisms, both the adhesion and friction forces of geckos can be changed over three orders of magnitude, allowing for the swift attachment and detachment during gecko motion. The results have obvious implications for the fabrication of dry adhesives and robotic systems inspired by the geckos locomotion mechanism.

Measurements of and Relation Between the Adhesion and Friction of Two Surfaces Separated by Molecularly Thin Liquid Films

A new technique is described for sliding (shearing) two molecularly smooth surfaces laterally past each other in liquids while monitoring their exact contact area, the normal and transverse forces, and the surface separation. First, we show that the elastic deformations of two initially curved surfaces in adhesive contact are the same under static and dynamic (i.e., sliding) conditions. Detailed results are then presented of how the shear properties of thin films of water and a simple nonpolar liquid are “quantized” with the number of layers. Results with water as the intervening liquid, as well as the effects of humidity on sliding in air, reveal that more complex mechanisms are operating than with simple liquids which appear to be related to the complex “hydration” forces between two surfaces in water or in aqueous salt solutions. The results suggest a close correlation between the static forces and shear properties of very thin liquid films, and the molecular structure of the liquids confined within such films.

Adhesion and short-range forces between surfaces. Part I: New apparatus for surface force measurements

A new miniature Surface Forces Apparatus (SFA Mark III) is described for measuring the forces between surfaces in vapors and liquids. The apparatus employs similar techniques to those used in current SFAs, but it is easier to operate and is generally more user-friendly. Four stages of increasingly sensitive distance controls replace the three control stages of previous apparatuses. The first three stages allow for rapid manual control of surface separation to within 10 A, while the fourth piezo-control stage has a sensitivity of better than 1 A. All four distance controls have been specially designed to produce perfectly linear displacements of the surfaces. In addition, the SFA Mk III is more robust, less susceptible to thermal drifts, easier to clean, and requires smaller quantities of liquid than conventional SFAs. The high performance of this new instrument is illustrated in the proceeding paper (Paper II) which describes the subtle effects of surface lattice mismatch on the oscillatory forces in water in the distance regime from 0 to 10 A.

Peel-Zone Model of Tape Peeling Based on the Gecko Adhesive System

A tape-peeling model based on the geometry of the peel zone (PZ) is derived to predict the peeling behavior of adhesive tapes at peel angles less than or equal to 90°. The PZ model adds an angle-dependent multiplier to the Kendall equation that takes into account the geometrical changes within the peel zone. The model is compared with experimental measurements of the peel force at different angles for a model tape and two commercial tapes, each with different bending moduli, stretch moduli, and adhesive strengths. Good agreement is found for a wide range of peel angles. The PZ model is also applied to the gecko adhesive system and predicts a spatula peel angle of 18.4° to achieve the adhesion forces reported for single setae. The PZ model captures the fact that adhesive forces can be significantly enhanced by peeling at an angle, thereby exploiting high friction forces between the detaching material and the substrate.

Forces between bilayers of cetyltrimethylammonium bromide in micellar solutions

A direct force-measuring technique has been used to study the interaction forces between adsorbed CTAB (cetyltrimethylammonium bromide) bilayers at concentrations well above the CMC (critical micelle concentration). An analysis of these results based on the Poisson-Boltzmann equations leads to the conclusion that CTAB micelles and adsorbed bilayers are about 22(±4)% dissociated. The apparent agreement of bilayer and micellar ion binding parameters raises an important challenge for theories of double-layer interactions. In addition, the double-layer decay lengths observed in these micellar solutions appear to be due entirely to the dissociated bromide and free CTA+ ions, with no apparent contribution from charged micelles.

Flow induced coating of fluoropolymer additives: Development of frustrated total internal reflection imaging

In the extrusion of linear low-density polyethylene (PE), fluoropolymer processing additives (PPAs) are used to eliminate the surface defect known as “sharkskin” by coating the die wall and inducing slip at the PPA/PE interface. We describe an in situ optical method for measurement of the coating thickness by exploiting the phenomenon of frustrated total internal reflection. By correlating the optical and pressure measurements, extrudate appearance and auxiliary experiments, we can elucidate the kinetics of the coating process. The PPA droplets first adsorb in the entrance region of the die and migrate under shear stress towards the capillary exit where they act to suppress sharkskin. We find that a uniform coating in the range of 25–60 nm is sufficient for sharkskin elimination. The steady state coating thickness near the exit ranges from 200 to 400 nm depending on the shear rate and concentration. We develop a mass balance model for calculation of the steady state coating thickness which compares well w...

Comparison of friction measurements using the atomic force microscope and the surface forces apparatus: the issue of scale

Results are presented of lateral force measurements using the atomic force microscope (AFM) and the surface forces apparatus (SFA). Two different probes are used in the AFM measurements; a sharp silicon nitride tip (radius R≈20 nm) and a glass ball (R≈15 μm). The lateral force is measured between the (silicon nitride or glass) probe and a mica surface which has been coated by a thin lubricant film. In the SFA, a thin lubricant film separates two molecularly smooth mica surfaces (R≈1 cm) which are slid relative to each other. Perfluoropolyether (PFPE) and polydimethylsiloxane (PDMS) were used as the lubricant films. In the SFA where the contact diameter is largest, the PFPE film shows much lower friction than PDMS. As the size of the probe decreases, the difference in the measured friction decreases. For sharp AFM tips, no clear distinction between the tribological properties of the films can be made. Hence, the measured coefficient of friction varies according to the length scale probed, at least for small dimensions.

Human astrocytes develop physiological morphology and remain quiescent in a novel 3D matrix

Astrocytes are the most abundant glial cells in the brain and are responsible for diverse functions, from modulating synapse function to regulating the blood-brain barrier. In vivo, these cells exhibit a star-shaped morphology with multiple radial processes that contact synapses and completely surround brain capillaries. In response to trauma or CNS disease, astrocytes become activated, a state associated with profound changes in gene expression, including upregulation of intermediate filament proteins, such as glial fibrillary acidic protein (GFAP). The inability to recapitulate the complex structure of astrocytes and maintain their quiescent state in vitro is a major roadblock to further developments in tissue engineering and regenerative medicine. Here, we characterize astrocyte morphology and activation in various hydrogels to assess the feasibility of developing a matrix that mimics key aspects of the native microenvironment. We show that astrocytes seeded in optimized matrix composed of collagen, hyaluronic acid, and matrigel exhibit a star-shaped morphology with radial processes and do not upregulate GFAP expression, hallmarks of quiescent astrocytes in the brain. In these optimized gels, collagen I provides structural support, HA mimics the brain extracellular matrix, and matrigel provides endothelial cell compatibility and was found to minimize GFAP upregulation. This defined 3D microenvironment for maintaining human astrocytes in vitro provides new opportunities for developing improved models of the blood-brain barrier and studying their response to stress signals.

A study of surfactant solution wetting on mica

Abstract The results of contact angle measurements on mica surfaces with aqueous dihexadecyldimethylammonium acetate (DHDAA) solution droplets at concentrations in the range 10 −6 to 10 −3 M are reported. At each concentration, four separate angles were measured: initial advancing, static advancing, initial receding and “equilibrated” receding. The advancing and receding angles both fell over a period of about one hour; the “static” and “equilibrated” angles were the values measured after this time. In the more concentrated solutions ( 7.5×10 −5 M ), this reduction can be attributed to bilayer formation whereas in dilute solutions the adsorbed surfactant molecules apparently either turn around to form an intercalated monolayer or are desorbed, but only at the three-phase line. This ‘perimeter effect’ occurs even for a droplet of water placed on an adsorbed, insoluble hydrophobic monolayer. A separate study of the solid—liquid interface confirms that an intercalated monolayer can indeed be formed at low DHDAA surfactant concentrations. A comparison of the properties of the solid—liquid and solid—vapour interfaces away from the three-phase line (TPL) suggests that in the microscopic movement of the TPL during measurement, the surface regions exposed or covered do not immediately convert to the equilibrium adsorbed state. The rate of this equilibration apparently depends on the monomer concentration of the surfactant and is an important factor in determining the observed contact angle.