Study Of Adhesion Mechanism Between Water Droplet/Bubbules And Patterned Surfaces

The hydrophobicity of mineral surface is a very important factor affecting the flotation effect,and the surface hydrophobicity will not only be affected by the surface chemistry,but also the surface morphology of minerals.Moreover,the actual mineral surface is mostly non-uniform and not smooth,so it is necessary to study the interaction of droplets,bubbles and rough surfaces in the process of adhesion.In order to reveal the adhesion mechanism of droplets,bubbles and patterned surfaces,regular single-structure surface topographies were fabricated on stable Polydimethylsiloxane(PDMS)solid surface in this study,including pore-patterned,pillar-patterned and groove-patterned surfaces.Through the combination of the DCAT microbalance system and the CCD camera,a direct measurement of the interaction forces of a variety of smooth surfaces and three microstructures(rough)surfaces and droplets in the process of adhesion was proceeded,and the measured force values were compared with the theoretical calculations.First,the validity of theoretical model for calculation of forces acting on droplets and smooth solid surfaces was verified.The interaction forces between the solid surface and the droplets are composed of two parts:surface tension(Fs)and capillary force(FL).Interaction forces between the smooth surfaces and the droplets in adhesion process can be calculated by the theoretical model.Secondly,based on the visualization of the three-phase contact lines,the theoretical model for calculating the forces of the droplets and the isotropic(micro-pore,micro-pillar)patterned surfaces was explored.Pore 55 and Pillar 510 were selected as representative samples of pore-patterned and pillar-patterned surfaces,respectively.Topography of three-phase contact line on the surfaces of different patterns was observed.The three-phase contact line on the pore-patterned surface is continuous and is relatively smooth.On the Pillar-Patterned surface,the three-phase contact line is discontinuous,and the depinning time of the contact line is very short,and the three-phase contact line is basically located at the middle of the top of the micro pillars.According to the morphology of three-phase contact lines on different patterned surfaces,the value of the normalized contact line length(?)was estimated,and the theoretical model was used to calculate the force acting on the patterned(rough)surfaces and the droplets.For the pore-patterned surface,when ? is equal to 1.OO,there is a good agreement between the experimental value and the calculated value.For the pillar-patterned surface,when the ? is equal to 0.90 and 0.86,respectively,the maximum adhesion force(Fmax)and pull-off force(Foff)values obtained in the test agree well with the calculated values.The pioneering research in this paper proposed a theoretical model for calculating the force acting on patterned surfaces and liquid,and surface solid fraction should not be taken into account in the theoretical model.Then,the theoretical model for the calculation of the forces acting on droplets and anisotropic(micro-groove)patterned surfaces was explored and the droplet anisotropic wetting characteristics were analyzed.When the calculated values of the interaction forces agree with the measured values for groove-patterned surfaces,the values of 82 are 0.75-0.76,indicating that the three-phase contact line is contorted in parallel direction.Directly visualization showed that the three-phase contact line did undergo a significant contortion.In the initial stage,the contact angle measured in vertical direction(??)was higher than the contact angle measured in parallel direction(??);at or near the Fmax point,the both angles are almost equal;on the contrary,0i is less than ?? near the Foff point,which meant that when three-phase contact line moved across parallel multiple groove structures,there would be a more significant pinning effect of the contact line.During water droplet spread,the diameter a was greater than b.Surprisingly,at the Fmax and Foff points,the measured droplet base diameters became almost the same in both directions.In the most stable state(Fmax point),the droplets tend to return to a symmetrical configuration,which is characterized by a circular base shape and a uniform apparent contact angle around the droplet.At the same time,the relationship between forces,free energy change,and various parameters was studied.?in,?max are important parameters for charactering the spreading force(Fin),Fmax,respectively.There is no definite correlation between the adhesion force of the smooth surfaces or patterned surfaces to droplet and the contact angle hysteresis,retention force.The interaction forces of the solid surfaces and the droplet exhibit a good positive linear correlation with the corresponding adhesion work,and it increases with the increase of the contact radius.The Fin between the pore-patterned surfaces and droplet decreases as the solid fraction decreases.The decrease of the solid fraction will make the adhesion force of the pore-patterned surface less than the adhesion force of the smooth PDMS surface(negative effect),and the gas volume change will increase the adhesion force of the pore-patterned surface(positive effect).The overlap of those two effects makes part of the adhesion force of the pore-patterned surface is larger than the adhesion force of the smooth PDMS surface.The interaction forces between the Pillar-Patterned and groove-patterned surfaces and the droplet are less than the forces between the smooth PDMS surface and droplet,and the forces decrease with the decrease of the solid fraction.The change of the free energy of the system during the spontaneous spreading of droplet on the patterned surfaces is linearly related to the work of spreading force.When pillar-patterned surfaces are in the Cassie-Baxter state,it shows strong hydrophobicity.However,when the surface wetting state is changed to the Wenzel state,Fin and Fmax increase,and the contact radius will also increase significantly.Finally,the adhesion characteristics and influencing factors of bubbles and patterned surfaces were studied.The adhesion times of Pore 550,Pore 520,Pore 510 and Pore 55 and the groove-patterned samples(G 55,G 510,G 520)with bubbles increase with the decrease of the solid fraction.The contact diameters of G 55,G 510,G 520 and bubbles are smaller than the contact diameter of the smooth PDMS sample and bubble,and the contact diameter decreases with the decrease of the solid fraction.Increasing the deformation of bubble can improve adhesion of solid surface to bubble.When the solid fractions of the pillar-patterned surfaces are approximately equal,reducing the dimensions of microstructures can effectively decrease adhesion between solid surface and droplet and improve the success rate of adhesion between bubble and surface.Macroscopic bubbles on solid surface cause a significant increase in the diameter of adhesion between solid surface and bubble.The microbubbles on the surface of Pillar 5 make the adhesion time between surface and bubble shorter and contact diameter larger.The surface wetting state plays an important role in the contact angle and adhesion of the pillar-patterned surfaces to bubbles.