Effect of Gravity on Wettability of Material Surface

Author:Liu Yong Ming

Supervisor:yin da chuan


Degree Year:2017





As one of the basic characteristic properties of solid surface,wettability plays an important role in many fields such as anti-corrosion coatings,self-cleaning,and fluid drag reduction.Therefore,it has attracted extensive research interest of material science and technology.However,controversy still exists as to whether the wetting of material surface is affected by gravity.Theoretically,many scientists have derived the Young’s equation based on thermomechanics of wetting,and pointed out that the contact angle depends only on physic-chemical nature of the solid,liquid and vapor,and is independent of gravity.The gravity may effect the shape of liquid-vapor interface,but not the contact angle.Experimentally,others have measured the contact angles under different gravitational levels and found that the contact angle depends on the gravitational level actually.In a word,there is a controversy between theory and experiment.Thus,it is necessary to investigate the effect of gravity on wetting of material surface.Studies about wetting in microgravity could provide necessary theoretical basis for practical applications.However,expensive costs and limited experimental chances made the measurement of contact angle difficult to study in space.At present,a large gradient high magnetic field superconducting magnet is thought as one of the best ground-based simulation techniques which can provide a stable and long-duration simulating microgravity environment.In this thesis,we develop and construct a new setup that is specifically designed for use in a large gradient magnetic field to measure the contact angle between a levitated droplet and a solid surface.The wettability of hydrophobic and hydrophilic surfaces in this special environment is also studied.The results show that the sessile drop is flattened by gravity in normal condition,but in simulated microgravity,the drop is visually spherical.The advancing and receding contact angles in the simulated microgravity environment are significantly larger than the control in the normal gravity condition.The magnetic field doesn’t effect the contact angle when the direction of magnetic field is parallel to the solid-liquid interface.It indicates that the contact angle may be effected by gravity.Studies of about wettability at different gravitational levels can help us to understand the relationship between wetting of solid surface and gravity,and construction of Young’s equation in the presence of gravitational field.In this thesis,a long arm(radius=3 m)centrifuge is designed and constructed,as well as a new setup that was specifically designed for use in this centrifuge to remote measure the contact angle using sessile drop method.The centrifuge can provide stable and long-durable 1 g8 g gravitational level.The wettability of9 solid-liquid-vapor systems were studied at 1 g,2 g,3 g,4 g,5 g,6 g,7 g,and 8 g.The results show that with the increase of gravitational level,the deformation of sessile drop become more serious,and the advancing and receding contact angle decrease.Although the line tensions of different solid-liquid-vapor systems are different,all of them are positive,and the magnitude of line tension determined ranges from 10-55 to 10-6 N.For a given solid-liquid-vapor system,the line tension is constant,and not depend on the gravitational level.The mechanisms of the effect of gravity on contact angle is also discussed in this study.A modified Young’s equation is derived by considering the balance of forces acting on three-phase contact line where the solid,liquid and vapor phase meet.Besides the quantities of classical Young’s equation,new equation includes the effect of line tension and gravity.When the drop is small,the effect of gravity can be negligible and the equation is identical with the equation that developed by others.When the drop is large(three-phase contact radius>2.53.0 mm),the effect of line tension can be negligible and the equation is the modified Young’s equation that only contains the effect of gravity.However,only after we obtained the line tension can this equation be solved.In case of the contact angle measured at1 g for low contact angle hysteresis surface was their Young’s contact angles,we calculate the contact angles which is under 1 g8 g according to modified Young’s equation.The results indicate that parts of calculated values are in good agreement with experimental values.In the end,the differences between the experimental results and the theoretical calculation are analyzed.