Machine Vision Based Measurement of Dynamic Contact Angles in Microchannel Flows

When characterizing flows in miniaturized channels, the determination of the dynamic contact angle is important. By measuring the dynamic contact angle, the flow properties of the flowing liquid and the effect of material properties on the flow can be characterized. A machine vision based system to measure the contact angle of front or rear menisci of a moving liquid plug is described in this article. In this research, transparent flow channels fabricated on thermoplastic polymer and sealed with an adhesive tape are used. The transparency of the channels enables image based monitoring and measurement of flow variables, including the dynamic contact angle. It is shown that the dynamic angle can be measured from a liquid flow in a channel using the image based measurement system. An image processing algorithm has been developed in a MATLAB environment. Images are taken using a CCD camera and the channels are illuminated using a custom made ring light. Two fitting methods, a circle and two parabolas, are experimented and the results are compared in the measurement of the dynamic contact angles.

DYNAMIC CONTACT STIFFNESS OF VIBRATING RIGID SPHERE CONTACTING SEMI-INFINITE TRANSVERSELY ISOTROPIC VISCOELASTIC SOLID

Dynamic contact stiffness at the interface between a vibrating rigid sphere and a semi-infinite transversely isotropic viscoelastic solid is investigated. An oscillating force superimposed onto a static compressive force in the vertical direction excites the vibration of a rigid sphere, which causes variable contact radius and contact pressure distribution in the contact region. The assumption of a sufficiently small oscillating force yields a dynamic contact-pressure distribution of a constant contact radius, which gives dynamic contact stiffness at the interface between the rigid sphere and the semi-infinite solid. Numerical calculations show the influence of vibration frequency of the sphere, and elastic constants of the transversely isotropic solid on dynamic contact stiffness, which benefits quantitative evaluation of elastic constants and orientation of single hexagonal grains by resonance-frequency shifts of the oscillator in resonance ultrasound microscopy.

Prediction of curved oil–water interface in horizontal pipes using modified model with dynamic contact angle

In this study,interface shapes of horizontal oil–water two-phase flow are predicted by using Young-Laplace equation model and minimum energy model.Meanwhile,the interface shapes of horizontal oil–water twophase flow in a 20 mm inner diameter pipe are measured by a novel conductance parallel-wire array probe(CPAP).It is found that,for flow conditions with low water holdup,there is a large deviation between the model-predicted interface shape and the experimentally measured one.Since the variation of pipe wetting characteristics in the process of fluid flow can lead to the changes of the contact angle between the fluid and the pipe wall,the models mentioned above are modified by considering dynamic contact angle.The results indicate that the interface shapes predicted by the modified models present a good consistence with the ones measured by CPAP.

Numerical and Experimental Study on Droplet Spreading Motion after Impingement Considering Dynamic Contact Angle with CCUP （CIP-Combined Unified Procedure） Method

The contact angle is one of important parameters to simulate droplet spreading and impingement phenomena on the surface. In the most numerical research, it is assumed constant value and it is implemented as boundary condition. However, contact angle is changed according to contact line velocity and time. Hence, for accurate simulation, dynamic contact angle which has various values as time elapsed is adopted. In the present study, the numerical analysis is performed on the droplet spreading phenomena considering dynamic contact angle function which is obtained from single droplet spreading experiment on the flat and bare surface. The CIP （cubic interpolated pseudo-particle） method by Yabe is used for analysis of interface between liquid and gas phases. The numerical results considering contact angle function which newly modeled as time and contact angle are compared with numerical results considering Hoffman＇s function and experimental data for range of Weber number which are 4.427 and 11.334. In contrast of numerical result considering Hoffman＇s function, the numerical result shows good agreement with experimental data as time elapsed in contact angle evolution, deformation of droplet spreading radius and height. Indeed, overall, the results display the increasing maximum spreading radius and the decreasing height as Weber numbers increased.

Analysis of the Correlation between the Level of Contact Degradation and the Dynamic Resistance Curve in Circuit Breakers

The DRM （dynamic contact resistance measurement） in high voltage circuit breakers is a manner of evaluating the internal ageing condition of the chamber. DRM is similar to static contact resistance measurement testing, but instead of measuring a single value when the breaker contacts are closed （static value）, the ohmic resistance is measured at various contact positions, from the beginning of the contact opening until a complete separation of the contacts. The relationship between the contact resistances of the new circuit breaker and the ageing circuit breaker in operation provides subsidy for the evaluation of both the main and arcing contact conditions. This research aims to analyze the correlation between the various levels of degradation of the contacts and the configuration of the DRM curve. This work considers curve samples from new acceleration tests. breaker chamber contacts and different levels of degradation by

Contact dynamics of elasto-plastic thin beams simulated via absolute nodal coordinate formulation

Under the frame of multibody dynamics, the contact dynamics of elasto-plastic spatial thin beams is numerically studied by using the spatial thin beam elements of absolute nodal coordinate formulation（ANCF）. The internal force of the elasto-plastic spatial thin beam element is derived under the assumption that the plastic strain of the beam element depends only on its longitudinal deformation.A new body-fixed local coordinate system is introduced into the spatial thin beam element of ANCF for efficient contact detection in the contact dynamics simulation. The linear isotropic hardening constitutive law is used to describe the elasto-plastic deformation of beam material, and the classical return mapping algorithm is adopted to evaluate the plastic strains. A multi-zone contact approach of thin beams previously proposed by the authors is also introduced to detect the multiple contact zones of beams accurately, and the penalty method is used to compute the normal contact force of thin beams in contact. Four numerical examples are given to demonstrate the applicability and effectiveness of the proposed elasto-plastic spatial thin beam element of ANCF for flexible multibody system dynamics.

A Perturbation Result for Dynamical Contact Problems

This paper is intended to be a first step towards the continuous dependence of dynamical contact problems on the initial data as well as the uniqueness of a solution. Moreover,it provides the basis for a proof of the convergence of popular time integration schemes as the Newmark method.We study a frictionless dynamical contact problem between both linearly elastic and viscoelastic bodies which is formulated via the Signorini contact conditions.For viscoelastic materials fulfilling the Kelvin-Voigt constitutive law,we find a characterization of the class of problems which satisfy a perturbation result in a non-trivial mix of norms in function space.This characterization is given in the form of a stability condition on the contact stresses at the contact boundaries.Furthermore,we present perturbation results for two well-established approximations of the classical Signorini condition:The Signorini condition formulated in velocities and the model of normal compliance,both satisfying even a sharper version of our stability condition.

Dynamics of Spreading of Liquid on Solid Surface

Based on assuming that there is the precursor film in the front of the apparent contact line （ACL）, a model was proposed to understand the dynamic wetting process and associated dynamic contact angle. The present model indicated that a new dimensionless characteristic parameter, 2, attects the dynamic wetting process and associated dynamic contact angle as well. However, the previous model suggested that the dynamic contact angle is dependent＇on the capillary number and static contact angle only. An experimental investigation was conducted to measure the dynamic wetting behavior of silicon oil moving over glass, aluminum and stainless steel surfaces. It concluded that when the value of 2 was selected as 0.07, 0.16 and 0.35 for glass, aluminum and stainless steel, respectively, the experimental results were in good accordance with the prediction of the model. Furthermore, the comparison of the model with Strom＇s experimental data showed that 2 is independent on the species of liquids. Apparently, 2 should be interpreted as the effect of the solid surface properties on the dynamic wetting process.Meanwhile, it is found in the present experiment that the Hoffman-Voinov-Tanner law, which is valid at very low capillary number （Ca 〈〈 1 or 80〈 10°） recommend by Cazabat, still holds for higher contact angles, even up to 70°-80°. This is explained by （he present model very well.

Universal dynamic scaling and Contact dynamics in quenched quantum gases

Recently universal dynamic scaling is observed in several systems,which exhibit a spatiotemporal self-similar scaling behavior,analogous to the spatial scaling near phase transition.The latter one arises from the emergent continuous scaling symmetry.Motivated by this,we investigate the possible relation between the scaling dynamics and the continuous scaling symmetry in this paper.We derive a theorem that the scaling invariance of the quenched Hamiltonian and the initial density matrix can lead to the universal dynamic scaling.It is further demonstrated both in a two-body system analytically and in a many-body system numerically.For the latter one,we calculate the dynamics of quantum gases quenched from the zero interaction to a finite interaction via the non-equilibrium high-temperature virial expansion.A dynamic scaling of the momentum distribution appears in certain momentum-time windows at unitarity as well as in the weak interacting limit.Remarkably,this universal scaling dynamics persists approximately with smaller scaling exponents even if the scaling symmetry is fairly broken.Our findings may offer a new perspective to interpret the related experiments.We also study the Contact dynamics in the BEC−BCS crossover.Surprisingly,the half-way time displays a maximum near unitarity while some damping oscillations occur on the BEC side due to the dimer state,which can be used to detect possible two-body bound states in experiments.

Sensitivity analyses of random cave groups on karst tunnel stability based on water–rock interaction using a novel contact dynamic method

This paper concentrates on the sensitivity and dynamic simulation of randomly distributed karst cave groups on tunnel stability and connectivity extended ratio based on water–rock interaction using a novel contact dynamic method(CDM).The concept of karst cave group connectivity extended ratio during tunneling and water inrush is proposed.The effects of cave shape and spatial distribution on Qiyueshan tunnel are investigated.Tunnel deformation and damage index,and connectivity extended ratio with uniform random karst cave groups are evaluated.The results demonstrate that the connectivity extended ratio is verified as a crucial judgment in predicting the safe distance and assessing the stability of the tunnel with the karst cave group.CDM model captures the fracture propagation and contact behavior of rock mass,surface flow,as well as the bidirectional water–rock interaction during the water inrush of Qiyueshan tunnel with multiple caves.A larger cave radius and smaller minimum distance between the cave and tunnel increase the deformation and damage index of the surrounding rock.When the cave radius and cave area ratio increase,the failure pattern shifts from overall to local failure.These findings potentially have broad applications in various surface and subsurface scenarios involving water–rock interactions.

An efficient formulation based on the Lagrangian method for contact–impact analysis of flexible multi-body system

In this paper,an efficien formulation based on the Lagrangian method is presented to investigate the contact–impact problems of f exible multi-body systems.Generally,the penalty method and the Hertz contact law are the most commonly used methods in engineering applications.However,these methods are highly dependent on various non-physical parameters,which have great effects on the simulation results.Moreover,a tremendous number of degrees of freedom in the contact–impact problems will influenc thenumericalefficien ysignificantl.Withtheconsideration of these two problems,a formulation combining the component mode synthesis method and the Lagrangian method is presented to investigate the contact–impact problems in fl xible multi-body system numerically.Meanwhile,the finit element meshing laws of the contact bodies will be studied preliminarily.A numerical example with experimental verificatio will certify the reliability of the presented formulationincontact–impactanalysis.Furthermore,aseries of numerical investigations explain how great the influenc of the finit element meshing has on the simulation results.Finally the limitations of the element size in different regions are summarized to satisfy both the accuracy and efficien y.

Excitation Prediction by Dynamic Transmission Error under Sliding Friction in Helical Gear System

Monte Carlo method was adopted to calculate the meshing error considering the manufacture error and assembly error of the meshing point along the time-varying contact line for helical gear pair. The flexural-torsion-axis dynamic model coupled was established under the tooth friction force and solved by the perturbation method to compute real dynamic tooth load. The change laws of the friction force and friction torque were obtained in a meshing period. The transmission error formulation was analyzed to introduce meshing excitations. The maximum dynamic transmission error, the maximum meshing force and the maximum dynamic factor were calculated under different speeds, external loads and damping factors. The conclusions can provide theoretical basis for the gear design especially in tooth profile correction.

Measurement of the friction coefficient of a fluctuating contact line using an AFM-based dual-mode mechanical resonator

A dual-mode mechanical resonator using an atomic force microscope （AFM） as a force sensor is developed. The resonator consists of a long vertical glass fiber with one end glued onto a rectangular cantilever beam and the other end immersed through a liquid-air interface. By measuring the resonant spectrum of the modified AFM cantilever, one is able to accurately determine the longitudinal friction coefficient ξv along the fiber axis associated with the vertical oscillation of the hanging fiber and the traversal friction coefficient ξh perpendicular to the fiber axis associated with the horizontal swing of the fiber around its joint with the cantilever. The technique is tested by measurement of the friction coefficient of a fluctuating （and slipping） contact line between the glass fiber and the liquid interface. The experiment verifies the theory and demonstrates its applications. The dual-mode mechanical resonator provides a powerful tool for the study of the contact line dynamics and the rheological property of anisotropic fluids.

Visualizing experimental investigation on gas-liquid replacements in a microcleat model using the reconstruction method

Cleats are the main channels for fluid transport in coal reservoirs.However,the microscale flow characteristics of both gas and water phases in primary cleats have not been fully studied as yet.Accordingly,the local morphological features of the cleat were determined using image processing technology and a transparent cleat structure model was constructed by microfluidic lithography using the multiphase fluid visualization test system.Besides,the effect of microchannel tortuosity characteristics on two‐phase flow was analyzed in this study.The results are as follows:(1)The local width of the original cleat structure of coal was strongly nonhomogeneous.The cleats showed contraction and expansion in the horizontal direction and undulating characteristics in the vertical direction.(2)The transient flow velocity fluctuated due to the structural characteristics of the primary cleat.The water‐driven gas interface showed concave and convex instability during flow,whereas the gas‐driven water interface presented a relatively stable concave surface.(3)The meniscus advanced in a symmetrical pattern in the flat channel,and the flow stagnated due to the influence of undulation points in a partially curved channel.The flow would continue only when the meniscus surface bypassed the stagnation point and reached a new equilibrium position.(4)Enhanced shearing at the gas-liquid interface increased the gas‐injection pressure,which in turn increased residual liquids in wall grooves and liquid films on the wall surface.

Transient study of droplet oscillation characteristics driven by an electric field

Electrowetting technology,a microfluidic technology,has attracted more and more attention in recent years and has broad prospects in terms of microdroplet drive.In this paper,the dynamic contact angle theory is used to develop a numerical model to predict the droplet dynamic contact behavior and internal flow field under electrowetting.In particular,based on the established computational model of droplet force balance,the dynamic process of a droplet under electrowetting is analyzed,including the perspective of pressure variation and force balance inside the droplet.The results show that when the alternating current frequency increases from 50 Hz to 500 Hz,the amplitude of the oscillation waveform after droplet stabilization is 0.036 mm,0.016 mm,0.013 mm and 0.002 mm,while the relevant droplet oscillation period T is 11 ms,4 ms,2 ms and 1 ms,respectively.It is also found that the initial phase angle does not affect the droplet oscillation amplitude.In addition,the pressure on the droplet surface under alternating current electrowetting increases rapidly to the maximum value with resonant waveform oscillation,and the droplet will present different resonance modes under voltage stimulation.The higher the resonance mode is,the smaller the droplet oscillation amplitude is and the streamline at the interface will present an eddy current,in which the number of vortices matches the resonance mode.A high resonance mode corresponds to a small droplet amplitude,while there are more vortices with a smaller size.

Mimicking a Superhydrophobic Insect Wing by Argon and Oxygen Ion Beam Treatment on Polytetrafluoroethylene Film

Biological tiny structures have been observed on many kinds of surfaces such as lotus leaves and insect wings,which enhance the hydrophobicity of the natural surfaces and play a role of self-cleaning.We presented the fabrication technology of a superhydrophobic surface using high energy ion beam.Artificial insect wings that mimic the morphology and the superhydrophobocity of cicada's wings were successfully fabricated using argon and oxygen ion beam treatment on a polytetrafluoroethylene (PTFE)film.The wing structures were supported by carbon/epoxy fibers as artificial flexible veins that were bonded through an autoclave process.The morphology of the fabricated surface bears a strong resemblance to the wing surface of a cicada,with contact angles greater than 160°,which could be sustained for more than two months.

Simulation solution for micro droplet impingement on a flat dry surface

This paper presents a computational fluid dynamics approach for micro droplet impacting on a flat dry surface. A two-phase flow approach is employed using FLUENT VOF multiphase model to calculate the flow distributions upon impact. The contact line velocity is tracked to calculate the dynamic contact angle through user defined function program. The study showed that the treatment of contact line velocity is crucial for the accurate prediction of droplet impacting on poor wettability surfaces. On the other hand, it has much less influence on the simulation of droplet impacting on good wettability surfaces. Good fit between simulation results and experimental data is obtained using this model.

Experimental and modeling study of wettability alteration through seawater injection in limestone:a case study

Water flooding is widely applied for pressure maintenance or increasing the oil recovery of reservoirs.The heterogeneity and wettability of formation rocks strongly affect the oil recovery efficiency in carbonate reservoirs.During seawater injection in carbonate formations,the interactions between potential seawater ions and the carbonate rock at a high temperature can alter the wettability to a more water-wet condition.This paper studies the wettability of one of the Iranian carbonate reservoirs which has been under Persian Gulf seawater injection for more than 10 years.The wettability of the rock is determined by indirect contact angle measurement using Rise in Core technique.Further,the characterization of the rock surface is evaluated by molecular kinetic theory(MKT)modeling.The data obtained from experiments show that rocks are undergoing neutral wetting after the aging process.While the wettability of low permeable samples changes to be slightly water-wet,the wettability of the samples with higher permeability remains unchanged after soaking in seawater.Experimental data and MKT analysis indicate that wettability alteration of these carbonate rocks through prolonged seawater injection might be insignificant.

Pantograph-catenary are test apparatus for high-speed railway

With the continuous increase of train speed,undulations of catenary and vibrations of the pantograph head result in generating pantograph- catenary arc frequently,intensifying the abrasion between pantograph strip and catenary wire,which has seriously influenced the current collection and safety of electric multi units(EMU). It is necessary to study the pantographcatenary arc in immediately. Some researchers develop a few pantograph- catenary arc testing equipment,which couldn’t really reflect the operating condition of pantograph-catenary system. In this paper,the pantograph-catenary arc test apparatus was developed,which simulated the flexible and straight contact of pantograph strip and catenary wire,based on the coupling relationship between pantograph and catenary. The equipment was used to research the electrical parameters of the pantograph-catenary arc and the dynamic contact resistance.

ANALYSIS OF AN ELASTO-PIEZOELECTRIC SYSTEM OF HEMIVARIATIONAL INEQUALITIES WITH THERMAL EFFECTS

In this paper we prove the existence and uniqueness of a weak solution for a dynamic electo-viscoetastic problem that describes a contact between a body and a foundation. We assume the body is made from thermoviscoelastic material and consider nonmonotone boundary conditions for the contact. We use recent results from the theory of hemivariational inequalities and the fixed point theory.