Wide-end fibers and their adhesion performance in biological attachment systems

Certain small creeping animals and insects generate the necessary adhesion force through the hairs of their feet. In addition to size, the shape of the fibrillar structures dramatically affects adhesion performance. One must develop a mathematical model to represent the relationship between the characteristics of fibers, surface roughness and adhesion quality. This paper provides a mathematical model and shows that wide-end fibrillar systems (WFS) are more compatible with variation in surface roughness. These systems adhere better than conventional systems with simple cylindrical fibers. The optimum geometry of fibers for maximum adhesion is discussed in this article. The obtained results are in good agreement with biological studies and are applicable to guide experimentation to achieve optimum artificial contacts.     

聚氨酯材料在负法向力下的摩擦性能研究

实验研究了4种聚氨酯材料的粘着及摩擦性能，目的是选择合适的材料来制造特种机器人的足掌，其设计以壁虎脚为仿生对象。结果表明，负法向力(即粘着力)随法向力的增加而呈对数规律增加；在负法向力下，切向力随负法向力的增加而线性增加；在一定负法向力下，切向力随初始法向力的增加按对数规律增加。     

How Compliance Compensates for Surface Roughness in Fibrillar Adhesion

Fibrillar interfaces play an important role in the ability of many small animals to adhere to surfaces. Surface roughness is generally deleterious to adhesion because it hinders the ability of mating surfaces to make contact, but fibrillar surfaces compensate for surface roughness by virtue of their enhanced compliance. We examine the relationship between roughness and compliance by analyzing the mechanics of detaching an array of fibrils from a substrate. The theory of Johnson, Kendall, and Roberts is used to describe the interfacial adhesion of each fibril, and roughness is modeled by making the fibril length a random variable subject to a probability distribution. We solve for the mean force response of a fibrillar array as a function of the displacement of the entire array. From these results we extract the mean fibrillar pull-off force and work to separate the fibrillar array and substrate. We show how the mean fibrillar pull-off force decreases with increasing roughness-height standard deviation: the relationship is linear for small height standard deviation, and the pull-off force trails off to zero for very rough surfaces. Conversely, the work of separation is shown to be unaffected by small roughness-height standard deviation, although it decreases toward zero for rougher surfaces. The effects of roughness may be offset by increasing fibrillar compliance; for small roughness-height standard deviation, we show that the reduction in pull-off force is inversely proportional to the normalized compliance. We also show that the work of separation increases linearly with the compliance when the compliance is large compared with the roughness-height standard deviation.     

How Compliance Compensates for Surface Roughness in Fibrillar Adhesion

Fibrillar interfaces play an important role in the ability of many small animals to adhere to surfaces. Surface roughness is generally deleterious to adhesion because it hinders the ability of mating surfaces to make contact, but fibrillar surfaces compensate for surface roughness by virtue of their enhanced compliance. We examine the relationship between roughness and compliance by analyzing the mechanics of detaching an array of fibrils from a substrate. The theory of Johnson, Kendall, and Roberts is used to describe the interfacial adhesion of each fibril, and roughness is modeled by making the fibril length a random variable subject to a probability distribution. We solve for the mean force response of a fibrillar array as a function of the displacement of the entire array. From these results we extract the mean fibrillar pull-off force and work to separate the fibrillar array and substrate. We show how the mean fibrillar pull-off force decreases with increasing roughness-height standard deviation: the relationship is linear for small height standard deviation, and the pull-off force trails off to zero for very rough surfaces. Conversely, the work of separation is shown to be unaffected by small roughness-height standard deviation, although it decreases toward zero for rougher surfaces. The effects of roughness may be offset by increasing fibrillar compliance; for small roughness-height standard deviation, we show that the reduction in pull-off force is inversely proportional to the normalized compliance. We also show that the work of separation increases linearly with the compliance when the compliance is large compared with the roughness-height standard deviation.     

Unified theory and guidelines on adhesion

A new approach was proposed to consolidate the many adhesion theories into one coherent concept. The maximum attractive force between two sets of molecules (adhesion strength) is derived from the Lennard–Jones potential function and calculated with measured bond length and bond energy. It leads to two criteria for strong adhesion: intimate molecular contact of closer than 9 Å (necessary condition), and maximum attractive force with minimum potential energy (sufficient condition). The criteria conform to the key elements of most prior adhesion theories. Seven prior adhesion theories and their relevance to these two criteria were briefly reviewed/discussed. In order to draw up a set of guidelines on adhesion and supplement the missing pieces of information, 21 model polymers of varied functionality were synthesized to study (1) the effect of polar groups on adhesion and (2) the effect of polymer conformations on adhesion. The results indicate that polar groups are more effective in polymer backbone than in side chains for promoting adhesion. The presence of both hydrogen donors and acceptors in the same backbone maximizes adhesion. True (active) solvents enhance adhesion, cosolvents (latent solvents) boost adhesion by inducing favorable conformation of polymers in solution, but thinners (diluents) reduce adhesion. The set of guidelines covered the effects of functional groups, solvent blends, pigment loadings, adhesion promotion, and adhesion loss.     

Hydrophobization of hair to improve the interfacial adhesion between hair and high-density polyethylene

Concerned about environmental pollution, and aware of the comfort that polyethylene provides for daily human life, this work sought to replace a percentage of high-density polyethylene (HDPE) with human or bovine hair. Hair is natural, abundant, light weight, non-toxic, and disposed of as garbage. The main disadvantage of natural composites is the interfacial adhesion. To increase the interfacial adhesion between hair and HDPE, stearic acid or oleic acid was chemically anchored on the hair surface, which leads to an improved contact angle hysteresis and hydrophobicity. Dynamic-mechanical properties of the composites were investigated focusing on the type of carboxylic acid used (stearic or oleic acid), hair length, hair type (human or bovine) and amount of hair used in the composite. Taking 40°C as a reference, using 15% of hair with a length of 1 ± 0.15 mm, the highest storage modulus value was obtained with HDPE with human hair modified with oleic acid, exceeding the value of the storage modulus of HDPE by 67.64%. Increasing storage modulus on composites indicates of interfacial interaction and chemical affinity improvement between hair and polyethylene.     

Behaviors of time-dependent and time-independent adhesion forces revealed using an AFM under different humidities and measuremental protocols

Adhesion forces between a tipless cantilever and a silicon wafer were measured on an atomic force microscope (AFM) at two relative humidities (RH). Experimental results show that the behaviors of the adhesion force are largely influenced by the RH and measurement protocols. The capillary contribution to the measured adhesion force is time-dependent at low RH (31±1%) and time-independent at high RH (52±1%) due to different equilibrium times of water bridges. The time-dependent adhesion force increases logarithmically with contact time until saturation is reached. The effects of loading force, approach velocity, retraction velocity and measurement number on the adhesion force can be explained by the logarithmic contact time dependence of the adhesion force. However, the time-independent adhesion force decreases with dwell time, and increases with retraction velocity. No dependence of the adhesion force on loading force and approach velocity is found. The adhesion force by consecutively measurements on the same location shows fluctuations, and the trend is unpredictable.     

Superhydrophobic gecko feet with high adhesive forces towards water and their bio-inspired materials

Functional integration is an inherent characteristic for multiscale structures of biological materials. In this contribution, we first investigate the liquid-solid adhesive forces between water droplets and superhydrophobic gecko feet using a high-sensitivity micro-electromechanical balance system. It was found, in addition to the well-known solid-solid adhesion, the gecko foot, with a multiscale structure, possesses both superhydrophobic functionality and a high adhesive force towards water. The origin of the high adhesive forces of gecko feet to water could be attributed to the high density nanopillars that contact the water. Inspired by this, polyimide films with gecko-like multiscale structures were constructed by using anodic aluminum oxide templates, exhibiting superhydrophobicity and a strong adhesive force towards water. The static water contact angle is larger than 150° and the adhesive force to water is about 66 μN. The resultant gecko-inspired polyimide film can be used as a "mechanical hand" to snatch micro-liter liquids. We expect this work will provide the inspiration to reveal the mechanism of the high-adhesive superhydrophobic of geckos and extend the practical applications of polyimide materials.     

A moisture absorption and adhesion study of the process of blending film using p‐alkylphenol‐resorcinol‐formaldehyde and rubber latex

p‐alkylphenol‐resorcinol‐formaldehyde‐latex (ARFL) films were prepared by co‐condensation of p‐alkylphenols and resorcinol with formaldehyde to generate modified phenolic resins, followed by blending with rubber latex, aging, and finally curing. The weight‐gain of the ARFL films and the tensile force of the coated fiberglass were studied under different temperatures and various humidities. The surfaces of the ARFL films were further analyzed by measuring the static contact angle and the findings were confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and X‐ray photoelectron spectroscopy (XPS) analysis. The adhesion between the coated fiberglass and neoprene rubber was evaluated using the H‐adhesion technique. The best hydrophobicity and the largest water contact angle were displayed on the surface of the p‐nonylphenol‐resorcinol‐formaldehyde‐latex (NRFL) film, with a weight‐gain percent that was 40.0% (wt %) lower and a static contact angle that was 22.6° more than that of the resorcinol‐formaldehyde‐latex (RFL) film. The NRFL‐coated fiberglass had a higher tensile force and H‐adhesion force than the RFL‐coated fiberglass. The shelf life of NRFL‐coated fiberglass can be raised significantly at 40°C and under 98% humidity. The mechanism of the dramatic drop in the tensile force of the coated fiberglass is also discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Measurement of the adhesion and removal forces of submicrometer particles on silicon substrates

In this study, the magnitude of the applied removal force and its relationship to the theoretical adhesion force is determined. The removal is done through the application of known hydrodynamic drag and lift forces on submicrometer particles. The hydrodynamic removal forces are then correlated with the removal percentage and the adhesion forces. A useful correlation that can be used to determine the adhesion force from the known applied removal force and the removal percentage is presented. Below 90% removal, the data indicate a linear relationship between the removal force and the removal percentage. The effects of time on the adhesion force and particle deformation are also presented.     

Surface Roughness and Particle Adhesion

One of the important mechanisms affecting particle adhesion is the geometry of the contact between the particle and the surface. Atomic force microscopy can measure both this geometry and the particle adhesion. The positional dependence of adhesion of a point probe to a variety of rough surfaces has been measured. The Derjaguin approximation predicts that the adhesion fluctuations are proportional to the surface curvature fluctuations, if the adhesion is dominated by long range forces. Atomic force microscopy adhesion maps have directly verified this linearity.     

Surface Roughness and Particle Adhesion

One of the important mechanisms affecting particle adhesion is the geometry of the contact between the particle and the surface. Atomic force microscopy can measure both this geometry and the particle adhesion. The positional dependence of adhesion of a point probe to a variety of rough surfaces has been measured. The Derjaguin approximation predicts that the adhesion fluctuations are proportional to the surface curvature fluctuations, if the adhesion is dominated by long range forces. Atomic force microscopy adhesion maps have directly verified this linearity.     

Functionally graded dry adhesives based on f ilm-terminated silicone foam

A dry functionally graded adhesive (FGA) based on f ilm-terminated silicone foam was developed utilizing the foam's capacity of absorbing a large amount of energy to deliver high adhesion. The fabrication technique is based simply on sugar cube templating of common elastomers followed by film termination of the elastomer cubes using the same material. Dependences of the pull-off adhesive force and energy release rate on the preload and foam thickness were systematically investigated through a series of axisymmetric indentation/de-bonding tests. The contribution of the foam backing layer to the overall compliance and adhesion was analysed and discussed. The developed elastic f ilm-terminated structure has remarkable enhancement in pull-off force and work of adhesion, which can be employed in the transport of delicate objects as demonstrated in the pick and place of a silicon wafer. Furthermore, the proposed foam-based FGAs can be readily detached from the adherent surface by applying a slight shearing deformation. The research findings provide insights in understanding the role of mechanical graded properties in adhesion and can have technical implications in the development of a simple but effective dry adhesive material for mounting and transporting objects by automated robotic devices.     

Evolution and level behavior of adhesion force by repeated contacts of an AFM colloid probe in dry environment

Adhesion forces between a colloid probe and some samples were consecutively measured at a single location with an atomic force microscope (AFM) in a dry environment. The outcomes show that the adhesion force depends considerably on material, contact history, and number and distribution of asperities within the contact zone. Generally, there are four different stages for the adhesion force with increasing measurement number: random behavior for the first several or tens of contacts, then increasing monotonically, later remaining stable, and finally decreasing. Usually, the measured data points are grouped into several levels. Moreover, the adhesion force jumps frequently between different levels and is more inclined to jump to a neighboring level. The level behavior was attributed to the fact that the real contact region is not exactly the same between successive measurements. The differences in the adhesion force between two neighboring levels are almost the same for one location. The adhesion force in some levels usually increases or decreases discontinuously after jumping to other levels and then jumping back. The magnitude and fluctuations of the adhesion force and the number of levels depend on the number and distribution of asperities in the contact zone.     

Mapping of adhesion forces on soil minerals in air and water by atomic force spectroscopy (AFS)

The adhesion force between an atomic force microscope (AFM) tip and sample surfaces, mica and quartz substrates, was measured in air and water. The force curves show that the adhesion has a strong dependence on both the surface roughness and the environmental conditions surrounding the sample. The variability of the adhesion force was examined in a series of measurements taken at the same point, as well as at different places on the sample surface. The adhesion maps obtained from the distribution of the measured forces indicated regions contaminated by either organic compounds or adsorbed water. Using simple mathematical expressions we could quantitatively predict the adhesion force behavior in both air and water. The experimental results are in good agreement with theoretical calculations, where the adhesion forces in air and water were mostly associated with capillary and van der Waals forces, respectively. A small long-range repulsive force is also observed in water due to the overlapping electrical double-layers formed on both the tip and sample surfaces.     

Temperature dependence of microscale adhesion force between solid surfaces using an AFM

The adhesion force between two solid surfaces is of great interest with the rapid development of micro–nanodevices and instruments. The effect of temperature on the microscale adhesion force has been studied by recording force–displacement curves with an atomic force microscope. A flat tip with a diameter ~1.73 μm was used to prevent wear. The adhesion force measurements were carried out under ambient conditions and in a nitrogen-filled glove box. The substrate temperature was varied between 30 and 200 °C. The results show that when the temperature is <200 °C, the influence of temperature on the normal spring constant of the cantilever can be ignored. In this temperature range, the adhesion force distribution for each temperature exhibits a Gaussian-like distribution under both situations. Under ambient conditions, the mean adhesion force first increases with the increase in temperature and reaches the maximum at ~100 °C. Then the adhesion force begins to decline slightly. At about 150 °C, the adhesion force decreases dramatically, and remains relatively stable at high temperatures. The increase in adhesion force is associated with the capillary force. The elevated temperature leads to larger and more numerous liquid bridges. The capillary nucleation, the diffusion of water molecules, and the flow of thin water film are all enhanced with the elevated temperature. However, in dry nitrogen, the mean adhesion force decreases with the increase in temperature. This trend is attributed to the broken van der Waals bonds.     

Pressure-sensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths

Adhesive behavior in blends of high molecular weight poly(N-vinyl pyrrolidone (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) has been studied using a 180° peel test as a function of PVP-PEG composition and water vapor sorption. Hydrophilic pressure-sensitive adhesives are keenly needed in various fields of contemporary industry and medicine, and the PVP-PEG blends, pressure-sensitive adhesion has been established to appear within a narrow composition range, in the vicinity of 36 wt% PEG, and it is affected by the blend hydration. Both plasticizers, PEG and water, behave as tackifiers (enhancers of adhesion) in the blends with glassy PVP. However, PEP alone is shown to account for the occurrence of adhesion, and the tackifying effect of PEG is appreciably stronger than that of sorbed water. Blend hydration enhances adhesion for the systems that exhibit an apparently adhesive type of debonding from a standard substrate (at PEG content less than 36 wt%), but the same amounts of sorbed water are also capable of depressign adhesion in the PEG-overloaded blends, where a cohesive mechanism of adhesive joint failure is typical. The PVP-PEG blend with 36% PEG couples both the adhesive and cohesive mechanisms of bond rupture (i.e., the fibrillation of adhesive polymer under debonding force and predominantly adhesive locus of failure). Blend hydration effect on adhesion has been found to be reversible. The micromechanics of adhesive joint failure for PVP-PEG hydrogels involves the fibrillation of adhesive polymer, followed by fibrils stretching and fracturing as their elongation attains 1000-1500%. Peel force to rupture the adhesive bond of PVP-PEG blends increases with increasing size of the tensile deformation zone, increasing cohesive strength of the material, and increasing tensile compliance of the material, obeying the well-known Kaelble equation, derived originally for conventional rubbery pressure-sensitive adhesives. The major deformation mode upon peeling the PVP-PEG adhesive from a standard substrate is extension, and direct correlations have been established between the composition behaviour of peel strength and that of the total work of viscoelastic strain to break the PVP-PEG films under uniaxial drawing. As a result of strong interfacial interaction with the PET backing film, the PVP-PEG adhesive has a heterogeneous two-layer structure, where different layers demonstrate dissimilar adhesive characteristics.     

Measurements and Factorial Analysis of Micron-Sized Particle Adhesion Force to Indoor Flooring Materials by Electrostatic Detachment Method

Airborne concentration of micron-sized particulate matter (PM) is an important index of indoor air quality. While human activity is considered the main reason causing indoor particle resuspension, theoretical particle adhesion force models give predictions of adhesion force much larger than the disturbance forces introduced by human activity. This work suggests that the imperfect contact between particles and surfaces can greatly reduce the adhesion bond. Electrostatic detachment method is used to measure the actual adhesion force distribution of micron-sized particles to such common indoor flooring materials as vinyl and rubber. Comparisons are made between the theoretical predictions and experimental measurements. Factorial experiments are also designed to study the influence of particle type, flooring type and contact time on particle adhesion force.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

The number of measurements of a surface’s topography and the expected variation in adhesion to the surface

Variations in roughness on a surface spawn variations in adhesion force between the surface and any particles that contact the surface. To fully characterize the adhesion that will be exhibited when a particle contacts any location on the surface, it is desirable to map the surface with nanoscale detail. Since it is impractical to make nanoscale roughness measurements over the entirety of a surface with a characteristic dimension on the order of centimeters, a relationship between the number of surface measurements and the likely variation in the expected adhesion force is similarly desirable. In this work, the predicted van der Waals force was used to describe the particle adhesion force. The bootstrap statistical method was employed to estimate the error associated with the predicted mean adhesion force between a smooth spherical particle and a rough surface as a function of the number of locations on the surface where the roughness was measured. Specifically, 40 atomic force microscope (AFM) topographical scans (5 × 5 μm) were taken of three different surfaces and used as model surface inputs to an existing van der Waals adhesion force simulator. The simulator described the expected adhesion force resulting from 1200 contacts between the smooth, spherical particle (10 μm diameter) and random locations on each scanned area. After analyzing the results using the bootstrap method, it was determined that the adhesion between the particle and 10–15 scanned areas (out of 40) optimizes the accuracy of the predicted adhesion with respect to the researcher’s labor.