Electro-dry-adhesion

This work presents novel conductive bioinspired dry adhesives with mushroom caps that enable the use of a synergistic combination of electrostatic and van der Waals forces (electro-dry-adhesion). An increase in shear adhesion bond strength of up to 2046% on a wide range of materials is measured when a maximum electrical field of 36.4 V μm(-1) is applied. A suction effect, due to the shape of the dry adhesive fibers, on overall adhesion was not noted for electro-dry-adhesives when testing was performed at both atmospheric and reduced pressure. Utilization of electrostatics to apply a preloading force to dry adhesive fiber arrays allows increased adhesion even after electrostatic force generation has been halted by ensuring the close contact necessary for van der Waals forces to be effective. A comparison is made between self-preloading of the electro-dry-adhesives and the direct application of a normal preloading pressure resulting in nearly the same shear bond strength with an applied voltage of 3.33 kV on the same sample.     

Profiles of liquid drops at the bottom of cylindrical fibers standing on flat substrates

Based on Carroll's derivation that describes a symmetric liquid drop sitting on an infinite cylindrical fiber and the shape of the drop, we have extended the derivation to describe a drop located at the bottom of cylindrical fibers standing on flat substrates. According to our derivation, the shape of the drop forms a bell as predicted by Carroll but is cut off by the flat substrate. This theoretical prediction was verified experimentally using water, ethylene glycol, and Kaydol drops on glass, nylon and polypropylene cylindrical fibers, and on polytetrafluoroethylene (PTFE) and polyester (PET) flat substrates. We found that only four parameters are required to obtain agreement between the theoretical shape and the observed shape: the drop volume, the fiber radius, the liquid-fiber contact angle, and liquid-flat substrate contact angle.     

Adhesion behaviors of human trophoblast cells by contact with endothelial cells

Although it is still not clear whether migratory trophoblasts reach the spiral arteries by migration within blood vessels against blood flow or by a mechanism of directional cell division/proliferation, this process involves the attachment and adhesion of trophoblasts to endothelial cells lining the blood vessel walls. This raises the possibility that the cell–cell contact with endothelial cells may regulate trophoblast cell adhesion behaviors according to the surrounding flow condition. To test this, the adhesion forces of early gestation human trophoblast cells (TCs) cultured on glass slides coated with type I rat collagen or cultured with human umbilical vein endothelial cells (HUVECs) were measured quantitatively using a micropipette aspiration technique. Then, the resistance of TCs co-cultured with HUVECs to flow-induced shear stress was assessed with a flow chamber technique. The results showed that the adhesion force of TCs to glass slides coated with collagen was positively correlated with the concentration of collagen. By contact with endothelial cells, the adhesion force and the resistance to shear stress for the TCs were significantly enhanced. The interdiction of integrin β1 interaction remarkably reduced the adhesion forces of TCs to endothelial cells, hence their resistance to shear stress. The results therefore suggest that the contacts of TCs with endothelial cells enhance the adhesion forces of human TCs, partially by regulating with the integrin β1 according to the flow condition (i.e., the shear stress) in such a way to prevent the TCs from being carried downstream by flowing blood.     

Effect of silane coupling agent on mechanical interfacial properties of glass fiber‐reinforced unsaturated polyester composites

A silane coupling agent, γ‐methacryloxypropyltrimethoxysilane, for the surface modification of glass fibers was varied between 0.1 and 0.8 wt %. To understand the role of interfacial adhesion of glass fiber/unsaturated polyester composites, contact angles of the silane‐treated glass fibers were measured by the wicking method on the basis of the modified Washburn equation with deionized water, diiodomethane, and ethylene glycol as testing liquids. As a result, silane‐treated glass fibers led to increased surface free energy, mainly because of their increased specific or polar component. The mechanical interfacial behaviors based on the interlaminar shear strength (ILSS) of the composites determined by short‐beam tests and the critical stress‐intensity factor (K_IC) were also improved in the case of silane‐treated composites. The surface free energy and the mechanical interfacial properties especially showed the maximum value in the presence of 0.4 wt % silane coupling agent. It revealed that the increase of a specific component of the surface free energy or hydrogen bonding between the glass fibers and the coupling agents plays an important role in improving the degree of adhesion at interfaces in a composite system. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 55–62, 2003     

Targeted particulate adhesion to cellulose surfaces mediated by bifunctional fusion proteins

The adhesion of particles to surfaces is an integral element in many commercial and biological applications. In this article, we report on the direct measurements of protein-mediated deposition and binding of particles to model cellulose surfaces. This system involves a family of heterobifunctional fusion proteins that bind specifically to both a red dye and cellulose. Amine-coated particles were labeled with a red dye, and a fusion protein was attached to these particles at various number densities. The strength of adhesion of a single particle to a cellulose fiber was measured using micropipette manipulation as a function of the specificity of the protein and its surface density and contact time. The frequency and force of adhesion were seen to increase with contact time in fiber experiments. The dynamics of adhesion of the functionalized particles to cellulose-coated glass slides under controlled hydrodynamic flow was explored using a flow chamber for two scenarios: detachment of bound particles and attachment of particles in suspension as a function of the shear rate and surface density of protein. Highly specific adhesion was observed. The critical shear rate for particle detachment was an increasing function of cellulose binding domain (CBD) density on particle surface. A rapid irreversible attachment of particles to cellulose was observed under flow. Using a family of proteins that were divalent for binding either the red dye or cellulose, we found that particle detachment occurred because of the failure of the cellulose-CBD bond. A comparison of fiber binding and particle detachment results suggests that forces of adhesion of particles to cellulose of up to 2 nN can be obtained with this molecular system through multiple interactions. This study, along with the adhesion simulations currently under development, forms the basis of particulate design for specific adhesion applications.     

Contact shape controls adhesion of bioinspired fibrillar surfaces

Following a recent bioinspired paradigm, patterned surfaces can exhibit better adhesion than flat contacts. Previous studies have verified that finer contact structures give rise to higher adhesion forces. In this study, we report on the effect of the tip shape, which was varied systematically in fibrillar PDMS surfaces, produced by lithographic and soft-molding methods. For fiber radii between 2.5 and 25 microm, it is found that shape exerts a stronger effect on adhesion than size. The highest adhesion is measured for mushroom-like and spatular terminals, which attain adhesion values 30 times in excess of the flat controls and similar to a gecko toe. These results explain the shapes commonly found in biological systems, and help in the exploration of the parameter space for artificial attachment systems.     

Stick slip contact mechanics between dissimilar materials: effect of charging and large friction

Measurements of the contact radius as a function of applied force between a mica surface and a silica surface (mica/silica) in air are reported. The load/unload results show that the contact radius generally increases with applied force. Because of the presence of charging due to contact electrification, both a short-range van der Waals adhesion force and longer-range electrostatic adhesive interaction contribute to the measured force. The results indicate that approximately 20% of the pull-off force is due to van der Waals forces. The contact radius versus applied force results can be fit to Johnson-Kendall-Roberts (JKR) theory by considering that only the short-range van der Waals forces contribute to the work of adhesion and subtracting a constant longer-range electrostatic force. Also, an additional and unexpected step function is superimposed on the contact radius versus applied force curve. Thus, the contact diameter increases in a stepped dependence with increasing force. The stepped contact behavior is seen only for increasing force and is not observed when symmetric mica/mica or silica/silica contacts are measured. In humid conditions, the contact diameter of the mica/silica contact increases monotonically with applied force. Friction forces between the surfaces are also measured and the shear stress of a mica/silica interface is 100 times greater than the shear stress of a mica/mica interface. This large shear stress retards the increase in contact area as the force is increased and leads to the observed stepped contact mechanics behavior.     

Tailoring normal adhesion of arrays of thermoplastic, spring-like polymer nanorods by shaping nanorod tips

The tip shape of contact elements in hairy adhesion systems is crucial for proper contact formation and adhesion enhancement. While submicrometer terminal contact elements show much better adhesion performance than their larger counterparts, shaping their tips so as to maximize normal adhesion has remained challenging. We prepared durable nanorod arrays consisting of stiff, highly entangled thermoplastic polymers with rationally shaped tips by replication of anodic aluminum oxide (AAO). Nanorod arrays with pancake-like tips showed pronounced normal dry adhesion already for small loading forces. For small loading forces, adhesion forces significantly exceeded the loading forces. Both the absence of hysteresis in force/displacement curves and the pronounced durability of the nanorods in series of repeated attachment/detachment cycles suggest that the nanorods behave like elastic springs. Experimental load-adhesion curves were reproduced with a modified Schargott-Popov-Gorb (SPG) model, assuming that contacts between probe and individual nanorods are sequentially formed with increasing indentation depth.     

Nanoscale high-frequency contact mechanics using an AFM tip and a quartz crystal resonator

The transmission of high-frequency shear stress through a microscopic contact between an AFM tip and an oscillating quartz plate was measured as a function of vertical pressure, amplitude, and surface properties by monitoring the MHz component of the tip's deflection. For dry surfaces, the transmission of shear stress is proportional to the vertical load across the contact. This provides a measure of the forces of adhesion between the substrate and the tip. When stretching soft polymeric fibers created by pulling on the surface of a pressure sensitive adhesive, the transmitted shear stress decreased linearly with extension over the entire range of pulling. This contrasts with the static adhesive force, which remained about constant until it discontinuously dropped at the point of rupture.     

Experimental Study of Fibrin/Fibrin-Specific Molecular Interactions Using a Sphere/Plane Adhesion Model

Fibrin, the biopolymer produced in the final step of the coagulation cascade, is involved in the resistance of arterial thrombi to fragmentation under shear flow. However, the nature and strength of specific interactions between fibrin monomers are unknown. Thus, the shear-induced detachment of spherical monodispersed fibrin-coated latex particles in adhesive contact with a plane fibrin-coated glass surface has been experimentally studied, using an especially designed shear stress flow chamber. A complete series of experiments for measuring the shear stress necessary to release individual particles under various conditions (various number of fibrin layers involved in the adhesive contact, absence or presence of plasmin, the main physiological fibrinolytic enzyme) has been performed. The nonspecific DLVO interactions have been shown to be negligible compared to the interactions between fibrin monomers. A simple adhesion model based on the balance of forces and torque on particles, assuming an elastic behavior of the fibrin polymer bonds, to analyze the experimental data in terms of elastic force at rupture of an elementary intermonomeric fibrin bond has been used. The results suggested that this force (of order 400 pN) is an intrinsic quantity, independent of the number of fibrin layers involved in the adhesive contact. Copyright 2001 Academic Press.     

Interfacial behavior between atmospheric-plasma-fluorinated carbon fibers and poly(vinylidene fluoride)

Atmospheric-plasma fluorination was used to introduce fluorine functionalities onto the surface of carbon fibers without affecting their bulk properties. The interfacial adhesion between atmospheric-plasma-fluorinated carbon fibers and poly(vinylidene fluoride) (PVDF) was studied by means of direct wetting measurements and single fiber pullout tests. Measured contact angles of PVDF melt droplets on modified carbon fibers show that short exposure times of carbon fibers to atmospheric-plasma fluorination (corresponding to a degree of surface fluorination of F/C = 0.01 (1.1%)) leads to improved wettability of the fibers by PVDF melts. The apparent interfacial shear strength as a measure of practical adhesion, determined by the single-fiber pullout test, increases by 65% under optimal treatment conditions. The improved practical adhesion is not due to the formation of transcrystalline regions around the fibers or a change of the bulk matrix crystallinity or to an increased surface roughness; it seems to be due to the compatibilization of the interface caused of the atmospheric-plasma fluorination of the carbon fibers.     

Orientation effects and rheology of short glass fiber-reinforced thermoplastics

The rheology of polyolefines, polyamide 6, and polystyrene-acrylonitrile filled with glass fibers of different concentrations and aspect ratios have been investigated in simple shear flow, capillary rheometry, and uniaxial elongation. A comparison is made with unfilled and glass bead-filled melts. Fiber orientation was investigated by X-ray microradiography.Steady-state viscosities are obtained on fibers aligned parallel to the direction of flow. Entrance pressure losses, the shape of the viscosity function, and the appearance of a yield stress are depending on the fiber aspect ratio. The temperature dependence of the viscosity function is not significantly different from that of the unfilled melt.Transient shear stresses were measured on samples of different initial orientations of the fibers. The change of fiber orientation during shear flow gives rise to a pronounced overshoot of shear stress and normal stress difference. Elastic strains in shear are increased by the fibers but elastic elongational strains are reduced. Mechanisms are proposed to explain the experimental observations.Dedicated to Dr. D. Heinze on the occasion of his 60th birthday.     

Enhanced wet adhesion and shear of elastomeric micro-fiber arrays with mushroom tip geometry and a photopolymerized p(DMA-co-MEA) tip coating

Using principles inspired by the study of naturally occurring sticky systems such as the micro- and nanoscale fibers on the toes of geckos and the adhesive proteins secreted by marine animals such as mussels, this study describes the development and evaluation of a novel patterned and coated elastomeric microfibrillar material for enhanced repeatable adhesion and shear in wet environments. A multistep fabrication process consisting of optical lithography, micromolding, polymer synthesis, dipping, stamping, and photopolymerization is described to produce uniform arrays of polyurethane elastomeric microfibers with mushroom-shaped tips coated with a thin layer of lightly cross-linked p(DMA-co-MEA), an intrinsically adhesive synthetic polymer. Adhesion and shear force characterization of these arrays in contact with a glass hemisphere is demonstrated, and significant pull-off force, overall work of adhesion, and shear force enhancements in submerged aqueous environments are shown when compared to both unpatterned and uncoated samples, as well as previously evaluated patterned and coated arrays with differing geometry. Such materials may have potential value as repeatable adhesives for wet environments, such as for medical devices.     

Mechanics of cell spreading within 3D-micropatterned environments

Most tissue cells evolve in vivo in a three-dimensional (3D) microenvironment including complex topographical patterns. Cells exert contractile forces to adhere and migrate through the extracellular matrix (ECM). Although cell mechanics has been extensively studied on 2D surfaces, there are too few approaches that give access to the traction forces that are exerted in 3D environments. Here, we describe an approach to measure dynamically the contractile forces exerted by fibroblasts while they spread within arrays of large flexible micropillars coated with ECM proteins. Contrary to very dense arrays of microposts, the density of the micropillars has been chosen to promote cell adhesion in between the pillars. Cells progressively impale onto the micropatterned substrate. They first adhere on the top of the pillars without applying any detectable forces. Then, they spread along the pillar sides, spanning between the elastic micropillars and applying large forces on the substrate. Interestingly, the architecture of the actin cytoskeleton and the adhesion complexes vary over time as cells pull on the pillars. In particular, we observed less stress fibers than for cells spread on flat surfaces. However, prominent actin stress fibers are observed at cell edges surrounding the micropillars. They generate increasing contractile forces during cell spreading. Cells treated with blebbistatin, a myosin II inhibitor, relax their internal tension, as observed by the release of pillar deformations. Moreover, cell spreading on pillars coated with ECM proteins only on their tops are not able to generate significant traction forces. Taken together, these findings highlight the dynamic relationship between cellular forces and acto-myosin contractility in 3D environments, the influence of cytoskeletal network mechanics on cell shape, as well as the importance of cell-ECM contact area in the generation of traction forces.     

Direct measurement of the binding force between microfabricated particles and a planar surface in aqueous solution by force-sensing piezoresistive cantilevers

We propose a force measurement method for evaluating the binding force between microscale flat surfaces in an aqueous solution. Using force-sensing piezoresistive cantilevers with sub-nanonewton force resolution, we have directly measured binding forces between SiO2-SiO2 microcontacts, which were created by gravity-driven random collision between microfabricated SiO2 cylindrical particles and a planar SiO2 substrate in a HCl solution. First, to examine our method we measured the pH dependence of the binding force. The binding forces were 12 and 5.8 nN at pH 1.0 and 2.0, respectively. As the pH increased, the binding force decreased and became zero at pH greater than 3.0. We confirmed that the bindings were based on the van der Waals' (VDW) force at pH 2.0 or less whereas a repulsive double-layer force acted between the surfaces at pH 3.0 or more. Second, the binding forces were categorized into a friction force or an adhesion force between the particles and the substrate. In the measurement, the friction force between the particle and the substrate was measured in the case when the particle slid on the substrate. On the contrary, the adhesion force was measured when the particle came off the substrate. Whether the particle slid or came off depended on the aspect ratio of the particle. We fabricated cylindrical particles with an aspect ratio of 0.03-2.0 and distinguished the friction force from the adhesion force by changing the aspect ratio of the particles. As a result, the friction force per unit contact area between SiO2-SiO2 flat surfaces was found to be 330 pN/microm2 +/- 20% when we used particles with a low aspect ratio (<0.1), and the adhesion force per unit contact area was 90 pN/microm2 +/- 20% for particles with a high aspect ratio (>0.4). For fluidic self-assembly that utilizes microscale surface contact in a liquid, our measurement method is an effective tool for studying and developing systems.     

Estimation of surface free energies and hamaker constants for fibrous solids by wetting force measurements

Wetting force at three-phase line was measured by the Wilhelmy technique using fibrous solids/liquid/liquid systems. Advancing and receding contact angles were calculated from the wetting forces during fiber immersion and emersion. The obtained results showed that contact angle hysteresis was due to the heterogeneity of the fiber surfaces. The dispersive and polar components of surface free energies of the fibers were determined from the advancing and receding contact angles, respectively. The Hamaker constants of the fibers were estimated from the dispersive components of their surface free energies.     

Interactions between crossed hair fibers at the nanoscale

The atomic force microscope fiber probe is used to directly measure the forces and friction between two human hairs under various conditions. It is shown that the forces between the hair fibers in solution can be well explained by a DLVO interaction and that cationic surfactant modifies the interactions in a manner entirely consistent with current views of adsorption behavior. A Coulombic attraction occurs between the crossed hair fibers in air due to the heterogeneity of the surface, and at shorter separations a clear dispersion interaction is observed. Exposure of the hair to a bleaching solution leads to the removal of the adhesion and solely a double-layer interaction. Two crossed hair fibers obey Amontons' classic law of friction, with a linear relation between applied load and frictional force, allowing the determination of a friction coefficient; positively charged surfactant adsorption is shown to reduce the friction coefficient between the fibers in a manner consistent with boundary lubrication by a palisade layer.     

Structure and nanomechanical characterization of electrospun PS/clay nanocomposite fibers

Electrospinning has been emerging as one of the most efficient methods to fabricate polymer nanofibers. In this paper, PS/clay nanocomposite fibers with varying diameters were electrospun onto solid substrates. The fiber diameters were adjusted from 4 microm to 150 nm by changing the solution concentration. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were used to characterize the fiber morphology. Shear modulation force microscopy (SMFM) was utilized to investigate the surface nanomechanical properties of electrospun fibers as a function of the fiber diameter and temperature. In the absence of clay, no change in T(g) was observed, even though a large increase of shear modulus below the glass transition temperature was found. This effect was postulated to result from the molecular chain alignment during electrospinning. The addition of functionalized clays to the spinning solution produced fibers with a highly aligned montmorillonite layer structure at a clay concentration of 4 wt %. Clay agglomerates were observed at higher concentrations. The existence of clay further enhanced the shear modulus of fibers and increased the glass transition temperature by nearly 20 degrees C.     

Effect of contact geometry on the pull-off force evaluated under high-vacuum and humid atmospheric conditions

The effect of condensed water on pull-off forces under high vacuum (HV) and 0 to 83% relative humidity (RH) N2 atmospheric conditions was evaluated for different contact geometries using atomic force microscopy (AFM). The pull-off force was measured using two types of contact geometry: contact between hemispherical asperities and a flat silicon probe on an AFM cantilever (called a spherical-flat contact) and between a flat silicon substrate and a flat nickel probe on an AFM cantilever (called a flat-flat contact). The hemispherical asperities were fabricated using a focused ion beam (FIB) system, and each peak had a radius of curvature of between 70 and 610 nm. The flat nickel probe was fabricated by friction-induced wear. Measurement results showed that for the spherical-flat contact the pull-off force was proportional to the radius of curvature of the asperity peak and was slightly lower in HV than in humid 14% RH N2. For the flat-flat contact in HV, with increasing contact time, the pull-off force increased in HV but decreased in humid 62 and 83% RH N2. The pull-off force in HV was lower than that in humid N2 when the contact time was less than 10 s but was higher when the contact time was longer than 30 s. The estimated adhesion force based on the Laplace pressure from the contact geometry agreed reasonably well with the measured pull-off force.     

Enhancement of capillary forces by multiple liquid bridges

Capillary forces can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. In this paper, we study numerically the effect of meniscus size on the force between two homogeneous flat plates for different contact angles. The force distance curves show excellent quantitative agreement with previous investigations. The results for n menisci of equal total liquid volume reveal interesting scaling properties and an unexpected maximum force for moderately hydrophilic surfaces (i.e., contact angles around 70 degrees ). Further, we calculate the minimum solid-liquid area for multiple bridges, the cohesive stress (i.e., force per area) between the plates, and the work required to separate them. The results are presented in two-dimensional maps, which may be useful in the understanding of biological attachment structures and in the design of artificial contact systems.