Mechanics of bio-inspired attachment systems contacting with rough surface: effect of spring orientation angle on hierarchical spring model

It is known that due to the asymmetrically designed structure of seta, gecko makes an attachment at 30° and detachment at 90° and achieves its detachment by changing orientation angle of seta from 30° to 90°. In this study, it is shown how pulling orientation affects pull-off force for gecko hierarchy using spring model during detachment from rough surface and then how to achieve easy and quick switch between attachment and detachment. This study adds the change of spring orientation angle to hierarchical spring model and extends it for determining the effect of orientation angle on the pull-off force. One-, two-, and three-level hierarchical models for gecko lamella consisting of seta, branches, and spatula contacting with the rough surface have been used. This study will provide us information about pull-off forces from rough surfaces and help to imitate and produce in particular gecko-like devices, reusable products and in general nanomaterials and nanodevices.     

Geckos’ foot hair structure and their ability to hang from rough surfaces and move quickly

Geckos generate the necessary adhesion force through their foot hair. The direction of the gecko's foot hair is not perpendicular to its finger surface, giving compliance to the hair. The effect of this compliance on the adhesion force is analysed and expressed theoretically in terms of contact mechanics. We conclude that the compliance of the foot hair is sufficient to generate the large adhesion force necessary for adhesion to rough surfaces, and that the structure of the seta with the spatulae allow the normal adhesion force to be controlled, allowing the gecko to make quick steps.     

Peeling of Elastic Tapes: Effects of Large Deformations,Pre-Straining,and of a Peel-Zone Model

A peel model for non-linear elastic tapes is presented which accounts for large deformations and for pre-straining. The large deformation setting is a new feature of modelling, which would be of interest for applications related to soft polymers and tissues. The conditions for having quasistatic-steady debonding or dynamic catastrophic debonding are determined in terms of the loading variables (peel angle and peeling force). The decohesion energy associated with a given process-zone model is included in the formulation of the peeling model. The predictions of various decohesion laws are discussed with respect to experimental results in the literature. Finally, the adhesion of a gecko is analysed and the maximum adhesion force of a single spatula is evaluated. The result correlates well with the maximum experimental pulling force reported in the literature for a gecko's seta.     

Prediction of bubble diameter at detachment from a wall orifice in liquid cross-flow under reduced and normal gravity conditions

Bubble formation and detachment is an integral part of the two-phase flow science. The objective of the present work is to theoretically investigate the effects of liquid cross-flow velocity, gas flow rate embodied in the momentum flux force, and orifice diameter on bubble formation and detachment in a wall-bubble injection configuration. A two-dimensional one-stage theoretical model based on a global force balance on the bubble evolving from a wall orifice in a cross liquid flow is presented in this work. In this model, relevant forces acting on the evolving bubble are expressed in terms of the bubble center of mass coordinates and solved simultaneously. Relevant forces in low gravity included the momentum flux, shear-lift, surface tension, drag and inertia forces. Under normal gravity conditions, the buoyancy force, which is dominant under such conditions, can be added to the force balance. Two detachment criteria were applicable depending on the gas to liquid momentum force ratio. For low ratios, the time when the bubble acceleration in the direction of the detachment angle is greater or equal to zero is calculated from the bubble x and y coordinates. This time is taken as the time at which all the detaching forces that are acting on the bubble are greater or equal to the attaching forces. For high gas to liquid momentum force ratios, the time at which the y coordinate less the bubble radius equals zero is calculated. The bubble diameter is evaluated at this time as the diameter at detachment from the fact that the bubble volume is simply given by the product of the gas flow rate and time elapsed. Comparison of the model's predictions was also made with predictions from a two-dimensional normal gravity model based on Kumar-Kuloor formulation and such a comparison is presented in this work.     

Surface properties of glass micropipettes and their effect on biological studies

In this paper, an investigation on surface properties of glass micropipettes and their effect on biological applications is reported. Pipettes were pulled under different pulling conditions and the effect of each pulling parameter was analyzed. SEM stereoscopic technique was used to reveal the surface roughness properties of pipette tip and pipette inner wall in 3D. More than 20 pipettes were reconstructed. Pipette heads were split open using focused ion beam (FIB) milling for access to the inner walls. It is found that surface roughness parameters are strongly related on the tip size. Bigger pipettes have higher average surface roughness and lower developed interfacial area ratio. Furthermore, the autocorrelation of roughness model of the inner surface shows that the inner surface does not have any tendency of orientation and is not affected by pulling direction. To investigate the effect of surface roughness properties on biological applications, patch-clamping tests were carried out by conventional and FIB-polished pipettes. The results of the experiments show that polished pipettes make significantly better seals. The results of this work are of important reference value for achieving pipettes with desired surface properties and can be used to explain biological phenomenon such as giga-seal formation.     

Modeling of Particle Layer Detachment under Consideration of Transient Kinetic Effects

Particle layers tend to build up on walls in many filtration and separation processes, calling for periodic removal in order to keep the device running. Important factors are the adhesion of the layer on the substrate and the cohesion of the particles in the layer. Models describing such layer detachment generally assume constant and homogeneous conditions for the forces acting on the layer. But in reality detachment is extremely nonstationary concerning place and time, primarily due to changing conditions for the forces on the one hand and changes in the particle layer morphology on the other. This paper describes a model and a simulation considering such transient kinetic effects on filter cake detachment. Diverse computing results are presented and discussed.     

Steady-State Mechanics of Delamination Cracking in Laminated Ceramic-Matrix Composites

A fracture mechanics delamination cracking model has been developed for brittle-matrix composite laminates. The near-tip mechanics is discussed in the context of material orthotropy and composite material inhomogeneities. A fracture mechanics framework based on the near-tip energy release rate and the associated phase angle Ψ has been adopted. In the case of steady-state delamination cracking in a prenotched cross-ply symmetric laminated beam, analytical expressions for the steady-state energy release rate, _ss, have been obtained for the combined applied loading of an axial force and a bending moment. Parameter studies assessing the effects on _ss of load coupling, crack location, and lamination morphology which includes the total number of layers, layer thickness, and material properties are presented. Thus, composite homogenization criteria with respect to the total number of layers placed along the beam height can be obtained for a wide range of material selection. The associated phase angle Ψ at the delamination crack tip is discussed in the context of existing solutions. The analysis has been developed based on a theory for structural laminates. The delamination model can be used in conjunction with experimental data obtained from model geometries to extract the mixed-mode transverse composite fracture toughness. Thus, conditions for stable delamination crack growth can be established and design criteria based on toughness for composite laminates and composite fasteners can be obtained.     

Mechanical Detachment of Nanometer Particles Strongly Adhering to a Substrate: An Application of Corrosive Tribology

The tip of a scanning probe microscope was used to detach nanometer-scale, single crystal NaCl particles grown on soda lime glass substrates. After imaging a particle at low contact forces, a single line scan at high contact force was used to detach the particle from the substrate. The peak lateral force at detachment is a strong function of particle contact area and humidity. As the relative humidity is raised from low to high values, the strength of the particle-substrate bond decreases dramatically. We interpret these results in terms of detachment by chemically-assisted crack growth along the NaCl-glass interface. Numerical estimates of the electrostatic and dispersive contributions to the work of adhesion are also discussed.     

Mechanical Detachment of Nanometer Particles Strongly Adhering to a Substrate: An Application of Corrosive Tribology

The tip of a scanning probe microscope was used to detach nanometer-scale, single crystal NaCl particles grown on soda lime glass substrates. After imaging a particle at low contact forces, a single line scan at high contact force was used to detach the particle from the substrate. The peak lateral force at detachment is a strong function of particle contact area and humidity. As the relative humidity is raised from low to high values, the strength of the particle-substrate bond decreases dramatically. We interpret these results in terms of detachment by chemically-assisted crack growth along the NaCl-glass interface. Numerical estimates of the electrostatic and dispersive contributions to the work of adhesion are also discussed.     

Microparticle detachment from surfaces exposed to turbulent air flow: controlled experiments and modeling

This work presents the results of experiments conducted to characterize the detachment of microparticles from surfaces exposed to turbulent air during accelerated free-stream flow. Smooth glass plates used as substrates are scanned with an atomic force microscope to determine their roughness-height distributions. Microparticles of different sizes, materials and shapes (mostly microspheres) are deposited as sparse monolayers onto the substrates under controlled clean and dry conditions. The microparticles attach to the substrate in a condition of static equilibrium due to adhesion and reside completely within the viscous sublayer as the flow is accelerated. Microvideographic observations of individual microparticle detachment show that detachment occurs primarily as rolling motion along the surface and not as lift-off. Detachment is not necessarily followed by entrainment in the flow. Results are presented as detachment fractions as function of time.The experimental results reveal that detachment is governed by a balance of the moments of aerodynamic drag and rough-surface pull-off forces. This is substantiated using a recently developed attachment theory that takes into account surface roughness to determine the pull-off force of microparticles. The sensitivity of the free-stream threshold velocity for detachment to five factors contained in the experiments and the model is analyzed. Results indicate that the surface energy of adhesion and the microsphere radius have the most influence on the threshold velocity for detachment.     

R-Curve Behavior of Long Cracks in Alumina

Coarse-grained alumina is among those monolithic ceramics which can exhibit an increase in crack resistance with crack extension. This R -curve behavior is most pronounced for intergranular fracture and does not depend exclusively on material properties. Crack and specimen geometries also influence the shape of the R -curves. The magnitude of the effect increases with increasing crack surface roughness, which is microstructure-dependent, and with crack-opening displacement, which is geometry-dependent. Based on experimental observations, a "dynamic" R -curve model is presented which relates the increasing resistance to an increasing crack tip shielding caused by crack surface bridging. Applying a J -integral approach, R -curves are calculated for two specimen geometries (short double cantilever beam and single-edged notched beam) and different grain sizes. The good agreement between calculation and experiment indicates that the R -curve behavior of long cracks in alumina can be predicted by a simple wake model.     

Formation of Small Bubbles in an Electric Field

Abstract

The formation of gas bubbles injected into an insulating liquid was studied under the influence of a nonuniform electric field. The electrode configuration used consisted of a charged capillary opposite a grounded ring, and gas bubbles were formed at the capillary tip. The experimental part of the investigation afforded insight into the mechanism of bubble formation, which is influenced by the existence of corona discharge. Based on the balance of forces at bubble detachment, a theoretical model was developed to predict the variation in bubble sizes. The theory agrees closely with experimental results over a range of applied voltages where the corona discharges were absent.     

PARTICLE REMOVAL MECHANISMS IN CRYOGENIC SURFACE CLEANING

Particle removal mechanisms in cryogenic surface cleaning are examined. The effect of impacting solid carbon dioxide particles and the hydrodynamic forces and torques are included in the model. Sliding detachment models and rolling detachment models in conjunction with the theory of critical moment are used. The critical conditions for removal of particles of different sizes are evaluated. For various carbon dioxide pellet diameters, incoming flow angles, nozzle, and cleaning surface separation distances, the critical shear velocities for particle detachment are evaluated.     

Adhesion analysis of two-level hierarchical morphology in natural attachment systems for 'smart adhesion'

The attachment ability of insects and lizards is well known. The Tokay gecko, in particular, has the most complex adhesion structures. The pads are covered by a large number of small hairs (setae) that contain many branches per seta with spatulae. Seta branch morphology is hierarchical. Hierarchical morphology of setae is responsible for adaptation of a large number of spatulae to rough surfaces. Van der Waals attraction between the large numbers of spatulae in contact with a rough surface is the primary mechanism for high adhesion. In order to investigate the effect of hierarchical structure, for the first time, the two-level hierarchical model has been developed. We consider one- and two-level hierarchically structured spring models for simulation of setae contacting with random rough surfaces and demonstrate the effect of the two-level hierarchical structure on the adhesion force, the number of contacts and the adhesion energy. Tip of spatula in a single contact was assumed as spherical. Rough surfaces with various roughness parameters which cover a common range of most of natural and artificial rough surfaces at the scale of gecko's pad were generated. It was found that significant adhesion enhancements are created with the two-level structure until a certain value of roughness which appears to be related to the maximum spring deformation. We conclude that the hierarchical morphology of a gecko seta is the necessary part for 'smart adhesion' of gecko, the ability to cling on and detach from different smooth, as well as rough surfaces.     

Sharpening of CVD diamond coated tools by 0.5−10 keV Ar ion beam

CVD diamond coated tungsten carbide tools have been used for cutting and drilling of soft materials such as aluminum and copper alloys. However, it is very difficult to obtain a tool having a sharp tip of the order of sub-μm by mechanical abrasive polishing methods. Therefore, we applied ion beam processing for sharpening the cutting edge of diamond coated tungsten carbide tools. Result shows that it is possible to obtain a 20-80 nm order tip width of a CVD diamond coated knife processed by a 0.5-10 keV Ar⁺ ion beam, and the sharpening speed of a tip of the knife depends on the ion beam energy. Namely, a tip width of a knife processes by a 1.0 keV Ar⁺ ion beam at an ion dose of 2.7 × 10²⁰ ions/cm² becomes 20 nm, and a tip width of a knife processed by a 10 keV Ar⁺ ion beam at an ion dose of 5.4 × 10¹⁹ ions/cm² becomes 40 nm. However, a facet with an apex angle in the range of 60-100° was formed on the cutting edge of a knife with an initial apex angle of 55°, and we found that the facet angle can be controlled by choosing ion beam energy of 0.5-10 keV. Moreover, results show that the processed knife machined by a 0.5 keV Ar⁺ ion beam has very smooth rake and flank faces, and also has a small line edge roughness of the cutting edge compared to those of the sharpened knife by a 1.0-10 keV Ar⁺ ion beam.     

Comparison of Cyclic and Monotonic Loading of a Double Cantilever Beam Adhesion Test

Double Cantilever Beam (DCB) specimens were made from aluminium plates bonded with Hysol®EA9321 epoxy adhesive. These were tested both under monotonic and cyclic loading conditions. Experimental testing data were obtained from classic force and displacement measurements, as well as from backface strain recordings. Apart from the usual crack propagation monitoring, evolution of the process zone at the crack tip was studied during the experiment. Results of fatigue were compared with those of standard loading. Despite the viscoelastic nature of the adhesive, an elastic treatment of the results proved most satisfactory. In addition, good agreement was found between experimental and theoretical results obtained with a Timoshenko beam on Winkler elastic foundation model.     

ANAEROBIC BIOFILM REACTOR MODELING FOCUSED ON HYDRODYNAMICS

This work deals with an experimental and theoretical investigation of anaerobic biofilm reactors for treating wastewaters. Bioreactors are modeled as dynamic (gas-solid-liquid) three-phase systems. The anaerobic digestion model proposed by Angelidaki et al. (1999) was selected to describe the substrate degradation scheme and was applied to a biofilm system. The experimental setup consists of two mesophilic (36°±1°C) lab-scale anaerobic fluidized bed reactors (AFBRs) with sand as inert support for biofilm development. The experimental protocol is based on step-type disturbances applied on the inlet substrate concentration (glucose and acetate-based feeding) and on the feed flow rate considering the criterion of maximum efficiency. The predicted and measured responses of biological and hydrodynamic variables are investigated. Experimental data were used to estimate empirical values of biofilm detachment coefficients. Under the evaluated operating conditions, the proposed model for biofilm detachment rate, assumed as a first-order function of the energy dissipation parameter, is appropriate to represent the interaction between biofilm systems and fluidization characteristics in non-highly disturbed flow conditions. Model validation was carried out using the experimental data reported by Mussati et al. (2006). The results do not differ from those above. This seems to indicate that the proposed AFBR model is able to reproduce the main biological and hydrodynamic successes in the bioreactor.     

Particle Removal in Linear Shear Flow: Model Prediction and Experimental Validation

In many industrial processes, particle contamination is becoming a major issue. Particle detachment from surfaces can be detrimental, e.g., during lithographic processing. During cleaning, however, detachment of particles is aimed for. However, until recently, only little was known on the mechanism of particle detachment due to flowing gasses. In high throughput applications, large gas velocities are likely to occur at certain locations in the system. It is important to test particle behavior experimentally under all conditions that may arise. Therefore, the aim of this study is to be able to predict the risk of particle detachment by modeling. For this purpose, particle–surface interaction is studied for micrometer-sized particles. Based on the particle Reynolds number, critical particle diameters were determined for which the flow-induced forces on the particles (drag and lift forces) are larger than the attractive forces between the particle and the surface (van der Waals force). Among the different possible particle motions (lift, sliding and rotation), particle rotation turns out to be the mechanism responsible for particle removal. A critical particle diameter was defined for which attractive and flow-induced forces are equal. Calculated values of the critical particle diameter agree with the experimental results within a few micrometers. This removal mechanism model can thus be used to calculate the cleaning efficiency of a flow, and for determining the probability of unwanted detachment of particles from surfaces in ultra-clean production or processing environments.     

AFM and TEM Studies of Pt Nanoparticle Arrays Supported on Alumina Model Catalyst Prepared by Electron Beam Lithography

Pt nanoparticle arrays were fabricated by electron beam lithography as a model for supported catalysts. The adhesion strength of 28 nm Pt nanoparticles deposited on alumina has been studied using contact mode atomic force microscopy. On as-prepared samples, the metal nanoparticles were removed by the AFM tip with a force of approximately 30 nN. After heat treatment at 500°C in a vacuum, Pt nanoparticles could not be removed by the AFM tip, even at 4000 nN. The increase of adhesion upon heat treatment indicates stronger bonding between Pt and the support. TEM examination showed that the Pt nanoparticles were polycrystalline before any treatment, with the crystalline domain increasing significantly after heat treatment.     

Positive and negative AC impedance feedback observed above conductive substrates under SECM conditions

A relationship between the tip response and the tip-to-substrate separation distance was investigated under alternating current scanning electrochemical microscopy (AC-SECM) conditions. For insulating substrates only negative AC feedback was observed. However, in the case of conductive substrates positive as well as negative AC feedback could be observed depending on experimental conditions. The theoretical model proposed in this work explains very well all observed trends and allows for qualitative predictions of the tip impedance in AC-SECM as a function of various experimental conditions (including: frequency, the supporting electrolyte concentration, the tip-to-sample separation distance and the tip diameter). In this theoretical model the double-layer capacitance of the tip and the sample is represented by a constant phase element (CPE), rather than an ideal capacitor. The paper also describes how raw experimental impedance data can be corrected for imperfections of the measuring system.