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.

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.

Theoretical Calculations and Experimental Verification for the Pumping Effect Caused by the Dynamic Micro-tapered Angle

The atomizer with micro cone apertures has advantages of ultra-fine atomized droplets, low power consumption and low temperature rise. The current research of this kind of atomizer mainly focuses on the performance and its application while there is less research of the principle of the atomization. Under the analysis of the dispenser and its micro-tapered aperture＇s deformation, the volume changes during the deformation and vibration of the micro-tapered aperture on the dispenser are calculated by coordinate transformation. Based on the characters of the flow resistance in a cone aperture, it is found that the dynamic cone angle results from periodical changes of the volume of the micro-tapered aperture of the atomizer and this change drives one-way flows. Besides, an experimental atomization platform is established to measure the atomization rates with different resonance frequencies of the cone aperture atomizer. The atomization performances of cone aperture and straight aperture atomizers are also measured. The experimental results show the existence of the pumping effect of the dynamic tapered angle. This effect is usually observed in industries that require low dispersion and micro- and nanoscale grain sizes, such as during production of high-pressure nozzles and inhalation therapy. Strategies to minimize the pumping effect of the dynamic cone angle or improve future designs are important concerns. This research proposes that dynamic micro-tapered angle is an important cause of atomization of the atomizer with micro cone apertures.

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.

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.

Unsteady aerodynamics modeling for flight dynamics application

In view of engineering application, it is practicable to decompose the aerodynamics into three components： the static aerodynamics, the aerodynamic increment due to steady rotations, and the aerodynamic increment due to unsteady separated and vortical flow. The first and the second components can be presented in conventional forms, while the third is described using a one-order differential equation and a radial-basis-function （RBF） network. For an aircraft configuration, the mathematical models of 6- component aerodynamic coefficients are set up from the wind tunnel test data of pitch, yaw, roll, and coupled yawroll large-amplitude oscillations. The flight dynamics of an aircraft is studied by the bifurcation analysis technique in the case of quasi-steady aerodynamics and unsteady aerodynam- ics, respectively. The results show that： （1） unsteady aerodynamics has no effect upon the existence of trim points, but affects their stability; （2） unsteady aerodynamics has great effects upon the existence, stability, and amplitudes of periodic solutions; and （3） unsteady aerodynamics changes the stable regions of trim points obviously. Furthermore, the dynamic responses of the aircraft to elevator deflections are inspected. It is shown that the unsteady aerodynamics is beneficial to dynamic stability for the present aircraft. Finally, the effects of unsteady aerodynamics on the post-stall maneuverability

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.

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.

Morphological Development of Fuel Droplets after Impacting Biomimetic Structured Surfaces with Different Temperatures

To improve the controllability for the evaporation process of fuel spray impinging on the cylinder wall,an experimental study on the development of morphological process of different fuel droplets on aluminium alloy surfaces is carried out.The metal surfaces with different wettability are prepared by laser etching and chemical etching for the experiments.In total,three different fuels are tested and compared under different surface temperatures,including diesel,n-butanol and dimethyl carbonate(DMC).The results show that under a lower wall temperature,the surface wettability,viscosity and surface tension of the fuels have significant effects on spreading and rebounding behaviour of the droplets.As the wall temperature rises over the boiling points of the fuel but below its Leidenfrost temperature,the contact angles between the fuels and surfaces are varying according to the surface wettability,boiling point and Leidenfrost temperature of the fuels.When the temperature of the surface exceeds the Leidenfrost temperature of all the fuels,after impacting the surfaces,different fuel droplets tend to have the same development pattern,regardless of the surface wettability.The rebound level is mainly affected by the amount of fuel vapour generated during the wall-hitting process.Viscosity,surface tension and other properties of the fuel have little effect on post-impacting behaviour of the droplet when the wall temperature is higher than the Leidenfrost temperature of the fuel.

Numerical analysis of frictional behavior of dense gas-solid systems

Dense gas-solid flows show significantly higher stresses compared with dilute flows, mainly attributable to particle-particle friction in dense particle flows. Several models developed have considered particle-particle friction; however, they generally underestimate its effect in dense regions of the gas-solid system, leading to unrealistic predictions in their flow patterns. Recently, several attempts have been made to formulate such flows and the impact of particle-particle friction on predicting flow patterns based on modified frictional viscosity models by including effects of bulk density changes on frictional pressure of the solid phase. The solid-wall boundary is also expected to have considerable effect on friction because particulate phases generally slip over the solid surface that directly affects particle-particle frictional forces. Polydispersity of the solid phase also leads to higher friction between particles as more particles have sustained contact in polydispersed systems. Their effects were investi- gated by performing CFD simulations of particle settlement to calculate the slope angle of resting material of non-cohesive particles as they settle on a solid surface. This slope angle is directly affected by frictional forces and may be a reasonably good measure of frictional forces between particles. The calculated slope angle, as a measure of frictional forces inside the system are compared with experimental values of this slope angle as well as simulation results from the literature.

A Level Set Method for the Simulation of Moving Contact Lines in Three Dimensions

We propose an efficient numerical method for the simulation of the twophase flows with moving contact lines in three dimensions.The mathematical model consists of the incompressible Navier-Stokes equations for the two immiscible fluids with the standard interface conditions,the Navier slip condition along the solid wall,and a contact angle condition(Ren et al.(2010)[28]).In the numerical method,the governing equations for the fluid dynamics are coupledwith an advection equation for a level-set function.The latter models the dynamics of the fluid interface.Following the standard practice,the interface conditions are taken into account by introducing a singular force on the interface in themomentum equation.This results in a single set of governing equations in the whole fluid domain.Similarly,the contact angle condition is imposed by introducing a singular force,which acts in the normal direction of the contact line,into theNavier slip condition.The newboundary condition,which unifies the Navier slip condition and the contact angle condition,is imposed along the solid wall.The model is solved using the finite difference method.Numerical results are presented for the spreading of a droplet on both homogeneous and inhomogeneous solid walls,as well as the dynamics of a droplet on an inclined plate under gravity.