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Yes / Tensioned web-roll coating is widely used but has surprisingly received little research attention. Here, a new semi-empirical model that predicts film transfer from applicator roller to web is developed and tested against data collected from a pilot coating line. The film transfer is found to vary linearly with web to applicator speed ratio S. Flow stability investigations revealed three types of defects: rivulets, air entrainment due to dynamic wetting failure and cascade, occurring at different values of S and capillary number Ca. Rivulets occurred at Ca< 0.4 and S> 0.71-0.81, air entrainment at Ca>0.4 and S>0.71-0.83 and cascades at S>1.1 for Ca up to 6. Web speeds at which dynamic wetting failure occurred were, for the same Ca, comparatively higher than those that occur in dip coating. The data show that such hydrodynamic assistance is due to the coating bead being confined, more so with increasing web wrap angle β. / The authors acknowledge the support of the Films R&D Centre of Toyobo Co. Ltd., Otsu, Japan and of the Thin Liquid Films Research Group of the University of Bradford, UK.

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Yes / Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.

Capillarity and dynamic wetting of non-Newtonian fluids are important in many natural and industrial processes, examples cover from a daily phenomenon as splashing of a cup of yogurt to advanced technologies such as additive manufacturing. The applicable non-Newtonian fluids are usually viscoelastic compounds of polymers and solvents. Previous experiments observed diverse interesting behaviors of a polymeric droplet on a wetted substrate or in a microfluidic device. However, our understanding of how viscoelasticity affects droplet dynamics remains very limited. This work intends to shed light on viscoelastic effect on two small scale processes, i.e., the motion of a wetting contact line and droplet splitting at a bifurcation tip.   Numerical simulation is employed to reveal detailed information such as elastic stresses and interfacial flow field. A numerical model is built, combining the phase field method, computational rheology techniques and computational fluid dynamics. The system is capable for calculation of realistic circumstances such as a droplet made of aqueous solution of polymers with moderate relaxation time, impacting a partially wetting surface in ambient air.   The work is divided into three flow cases. For the flow case of bifurcation tube, the evolution of the interface and droplet dynamics are compared between viscoelastic fluids and Newtonian fluids. The splitting or non-splitting behavior influenced by elastic stresses is analyzed. For the flow case of dynamic wetting, the flow field and rheological details such as effective viscosity and normal stress difference near a moving contact line are presented. The effects of shear-thinning and elasticity on droplet spreading and receding are analyzed, under inertial and inertialess circumstances. In the last part, droplet impact of both Newtonian and viscoelastic fluids are demonstrated. For Newtonian droplets, a phase diagram is drawn to visualize different impact regions for spreading, splashing and gas entrapment. For viscoelastic droplets, the viscoelastic effects on droplet deformation, spreading radius and contact line motion are revealed and discussed. / <p>QC 20160329</p>

In all coating applications, a liquid film displaces air in contact with a dry solid substrate. At a low substrate speed a thin uniform wetting line is formed on the substrates surface, but at a high speed the wetting line becomes segmented and unsteady as air becomes entrained between the substrate and the liquid. These air bubbles affect the quality of the coated product and any means to postpone this at higher speeds without changing the specifications of the coating liquid is desirable. This research assesses the validity of a theoretically based concept developed by Blake and Rushack [1] and exploited by Cohu and Benkreira [2] for dip coating. The concept suggests that angling the wetting line by an angle ß would increase the speed at which air is entrained by a factor 1/cos ß. In practice, if achieved this is a significant increase that would result in more economical operation. This concept was tested in a fast coating operation that of curtain coating which is already enhanced by what is known as hydrodynamic assistance [2]. Here we are effectively checking an additional assistance to wetting. The work, performed on a purposed built curtain coater and a rotating die, with a range of fluids showed the concept to hold but provided the data are processed in a way that separate the effect of curtain impingement from the slanting of the wetting line.

Wetting behavior of fluid/fluid/solid systems, largely influenced by surface properties and interactions between the three phases, plays a big role in nature and in industrial applications Traditionally, wetting studies have focused on liquid/vapor systems, especially the study of a sessile liquid droplet in air. Liquid/vapor systems can only probe the effects of surface properties and interactions between the solid and the wetting liquid. This type of characterization is inadequate for liquid/liquid systems, where surface wettability is additionally influenced by interactions between the two wetting liquids. The present study is the first to examine the effects of nanoscale roughness on wetting behavior in liquid/liquid systems and the modulation of roughness effects by fluid properties and the wetting order. This study examines both equilibrium and dynamic wetting behavior in liquid/liquid systems using well characterized substrates. Rough substrates were fabricated by coating glass substrates with nanometer sized polymer particles. Partial dissolution of the particles and molecular de-deposition of the polymer allowed for tuning of substrate roughness while retaining the original surface chemistry. The effectiveness of this fabrication technique was verified using electron microscopy and electrokinetic analysis. We examined the wetting behavior in three fluid/fluid systems: an air/water system, a decane/water system, and an octanol/water system. The oils were chosen based on their different polarities. Equilibrium wetting behavior was determined using contact angle measurements. Results indicate that for all systems where the primary wetting fluid was a liquid, an increase of the surface roughness resulted in Cassie-Baxter wetting. How hydrophilic a surface appears with regard to a water/fluid interface depended on the polarity of that fluid. The octanol/water system provided the strongest evidence regarding the effect of wetting order: a transition from Wenzel to Cassie-Baxter wetting was only observed when water was the primary wetting liquid. The observed transition was confirmed using a modified Wenzel/Cassie-Baxter model. The kinetics of droplet spreading was measured using high speed optical microscopy. After a droplet was placed on a solid surface, the motion of the contact line was imaged at a rate of 1000 fps. The wetted area was then extracted using custom Matlab® scripts. The spreading kinetics underwent a transition between two regimes: a visco-inertial regime and a slower spreading regime. Results indicated that surface roughness influenced spreading kinetics in both regimes. The overall spreading rate was always slower for rough surfaces than for smoother surfaces. In liquid/liquid systems, the duration of visco-inertial regime was dependent on the surface roughness as well; in general, it was shorter for smooth substrates compared to rough substrates. Increasing the viscosity of the non-aqueous fluid significantly increased the duration of the visco-inertial regime and decreased the overall spreading rate. This study provides insight into the competitive wetting of solid surfaces relevant in many industrial applications such as oil recovery or inkjet printing, and may guide the development of improved wetting models in an area that currently lacks an adequate theoretical description.

In the atmosphere in the height of 18000ft to 25000ft, there are some metastable droplets called supercooled liquid water in the temperature range of 0◦C to 40◦C. When these droplets impinge on the wings of an airplane, a very thin layer of ice is formed on the surface. This natural phenomenon calls “impact icing”. In this research, I studied the nanoscale impact icing on structured graphite surfaces, as the substrates at the atomistic scale using Molecular Dynamics (MD) simulations. This research focuses on the first steps of the development of a predictive multiscale strategy for molecular simulations of impact ice adhesion on nanostructured substrates. Through the simulations, the molecular level physics such as molecular interactions, interfacial energy, and nanoscale surface roughness are processed into a “microscopic ice adhesion strength” that describes the energy cost for breaking the nanoscale interfacial layer. In this work, the simulation strategy is designed based on the postulate that at the nanoscale the fracture strength of impact ice on a given substrate is controlled by the extent of the ice interdigitating the substrate. The interdigitating interfacial structure is then determined by the process of wetting the substrate by a supercooled impinged water droplet and the process of penetrating of supercooled water crystallizing into ice crystals under graphene nanoconfinement. Following this line of reasoning, I divided my impact icing simulations into three separate sections including (1) simulations of dynamic wetting of supercooled water on nanostructured graphene substrate, (2) simulations of water crystallization under nano-confinement, and (3) simulations of fracture of prescribed ice-substrate interfacial structure. Based on the results, it is concluded that the degree of surface hydrophobicity, depth of penetrated water, the order of interlocked water molecules, size of surface roughness, texture structure of the surface, and ice temperature are the key roles that dominate the investigation of fracture strength of impact ice at the solid interface. Furthermore, MD simulation results demonstrate that the surface roughness lower than 3.0nm is enabled to stop water from crystallization, a piece of useful information to design anti-icing surfaces.