The apparent contact angle of Cassie-Baxter state water droplets can be calculated by the existing theoretical formula, but due to the defects of the micro-structured hydrophobic surface and some inevitable tiny distu...The apparent contact angle of Cassie-Baxter state water droplets can be calculated by the existing theoretical formula, but due to the defects of the micro-structured hydrophobic surface and some inevitable tiny disturbances in the experiment, Cassie-Baxter state water droplets will appear partly in Wenzel state, that is, the mixed state water droplets. In this paper, apparent contact angles of Cassie-Baxter state and mixed state water droplets on micro-structured hydrophobic surfaces are compared. The research shows that if the projected area fraction of water-solid F in the Cassie-Baxter formula is replaced by the local projected area fraction of water-solid F′, the apparent contact angles of water droplets in both Cassie-Baxter state and the mixed state can be calculated. Further experimental results indicate that the contact state of water droplets nearby the outermost three-phase contact line plays a more important role in determining the apparent contact angle. This conclusion is significant to the understanding of the apparent contact angle and wetting property.展开更多
Wetting properties are significant for a hydrophobic surface and normally characterized by the equilibrium contact angle.In this manuscript,a mesoscopic method based on multiphase multiple-relaxation-time Lattice Bolt...Wetting properties are significant for a hydrophobic surface and normally characterized by the equilibrium contact angle.In this manuscript,a mesoscopic method based on multiphase multiple-relaxation-time Lattice Boltzmann method has been presented and applied to simulate the contact angle at three-phase interfaces of a solid surface with micro-pillar structure.The influence of different parameters including pillar height,pillar width,inter-pillar spacing,intrinsic contact angle and the volume of the liquid drop on the equilibrium contact angle has been comprehensively investigated.The effect of geometry parameters of the micro-pillar structure on the wetting transition from Cassie–Baxter state to Wenzel state has also been studied.The results indicate that when the inter-pillar spacing is less than a certain value or the pillar height is greater than a certain value,the contact form between the droplet and the surface satisfies the Cassie–Baxter state.When the contact form satisfies the Cassie–Baxter state,the contact angle gradually increases with the increase of the inter-pillar spacing;the contact angle does not change significantly with the pillar height;the contact angle gradually decreases and approaches the intrinsic contact angle with the pillar width increases.Moreover,the contact angle increases with the increase of the intrinsic contact angle,and the contact angle is not sensitive to the change of droplet volume when the droplet volume is between 0.5 and10μl.展开更多
A rapid prototyping process was presented to fabricate a nylon honeycomb microstructure coated with parylene C.The surface structure was designed to obtain a hydrophobic surface using the volume of fluid(VOF)model.Wit...A rapid prototyping process was presented to fabricate a nylon honeycomb microstructure coated with parylene C.The surface structure was designed to obtain a hydrophobic surface using the volume of fluid(VOF)model.With the micro-molding technique,the contact angle of the polymer surface could be designed and fabricated by changing the different microstructure surface diesteel mold inserts.For the honeycomb(20μm width and 60μm depth)microcavity side wall,an average micro-molding filling percentage of 95% could be achieved by using a three-section constant-pressure molding process.The solid surface wettability is governed by both the geometrical microstructures and the surface energy.A 2μm parylene C layer was deposited on the nylon honeycomb microsurface to reduce the surface energy.To design honeycomb structures with different microcavity densities,the contact angle of these artificial surfaces could change from 91°to 130°.From a comparison of the contact angle measurements with the different models,the honeycomb-structured microsurface could be described by the Cassie–Baxter model.The errors between the VOF simulation and the measured data were<10%.The drag reduction performance of the honeycomb microplates was investigated in a water tunnel with a high Reynolds number(from 0.5×10^(6) to 4.6×10^(6)).As a result,the honeycomb microplates showed a maximum drag reduction rate of 36±0.6%in comparison with the bare plates in such turbulent flow.Benefiting from the replaceable mold insert,more designable microstructure polymer surfaces can be manufactured by this rapid prototyping technique.展开更多
基金supported by the National Natural Science Foundation of China (Grant Nos. 10672089 and 10872106)
文摘The apparent contact angle of Cassie-Baxter state water droplets can be calculated by the existing theoretical formula, but due to the defects of the micro-structured hydrophobic surface and some inevitable tiny disturbances in the experiment, Cassie-Baxter state water droplets will appear partly in Wenzel state, that is, the mixed state water droplets. In this paper, apparent contact angles of Cassie-Baxter state and mixed state water droplets on micro-structured hydrophobic surfaces are compared. The research shows that if the projected area fraction of water-solid F in the Cassie-Baxter formula is replaced by the local projected area fraction of water-solid F′, the apparent contact angles of water droplets in both Cassie-Baxter state and the mixed state can be calculated. Further experimental results indicate that the contact state of water droplets nearby the outermost three-phase contact line plays a more important role in determining the apparent contact angle. This conclusion is significant to the understanding of the apparent contact angle and wetting property.
基金The authors are grateful for the financial support from the National Natural Science Foundation of China(Grant no.12172377and Grant no,11772351)theOpen Research Fund of Key Laboratory of Construction and Safety of Water Engineering of the Ministry of Water Resources,China Institute of Water Resources and Hydropower Research(Grant no.202007).
文摘Wetting properties are significant for a hydrophobic surface and normally characterized by the equilibrium contact angle.In this manuscript,a mesoscopic method based on multiphase multiple-relaxation-time Lattice Boltzmann method has been presented and applied to simulate the contact angle at three-phase interfaces of a solid surface with micro-pillar structure.The influence of different parameters including pillar height,pillar width,inter-pillar spacing,intrinsic contact angle and the volume of the liquid drop on the equilibrium contact angle has been comprehensively investigated.The effect of geometry parameters of the micro-pillar structure on the wetting transition from Cassie–Baxter state to Wenzel state has also been studied.The results indicate that when the inter-pillar spacing is less than a certain value or the pillar height is greater than a certain value,the contact form between the droplet and the surface satisfies the Cassie–Baxter state.When the contact form satisfies the Cassie–Baxter state,the contact angle gradually increases with the increase of the inter-pillar spacing;the contact angle does not change significantly with the pillar height;the contact angle gradually decreases and approaches the intrinsic contact angle with the pillar width increases.Moreover,the contact angle increases with the increase of the intrinsic contact angle,and the contact angle is not sensitive to the change of droplet volume when the droplet volume is between 0.5 and10μl.
基金The research was granted by the Chinese National Nature Science Foundation(Grant No.41571309).
文摘A rapid prototyping process was presented to fabricate a nylon honeycomb microstructure coated with parylene C.The surface structure was designed to obtain a hydrophobic surface using the volume of fluid(VOF)model.With the micro-molding technique,the contact angle of the polymer surface could be designed and fabricated by changing the different microstructure surface diesteel mold inserts.For the honeycomb(20μm width and 60μm depth)microcavity side wall,an average micro-molding filling percentage of 95% could be achieved by using a three-section constant-pressure molding process.The solid surface wettability is governed by both the geometrical microstructures and the surface energy.A 2μm parylene C layer was deposited on the nylon honeycomb microsurface to reduce the surface energy.To design honeycomb structures with different microcavity densities,the contact angle of these artificial surfaces could change from 91°to 130°.From a comparison of the contact angle measurements with the different models,the honeycomb-structured microsurface could be described by the Cassie–Baxter model.The errors between the VOF simulation and the measured data were<10%.The drag reduction performance of the honeycomb microplates was investigated in a water tunnel with a high Reynolds number(from 0.5×10^(6) to 4.6×10^(6)).As a result,the honeycomb microplates showed a maximum drag reduction rate of 36±0.6%in comparison with the bare plates in such turbulent flow.Benefiting from the replaceable mold insert,more designable microstructure polymer surfaces can be manufactured by this rapid prototyping technique.
