Understanding and predicting the behavior of water, especially in contact with various surfaces, is a scientific challenge. Molecularlevel understanding of hydrophobic effects and their macroscopic consequences, in particular, is critical to many applications. Macroscopically, a surface is classified as hydrophilic or hydrophobic depending on the contact angle formed by a water droplet. Because hydrophobic surfaces tend to cause water slip whereas hydrophilic ones do not, the former surfaces can yield self-cleaning garments and ice-repellent materials whereas the latter cannot. The results presented herein suggest that this dichotomy might be purely coincidental. Our simulation results demonstrate that hydrophilic surfaces can show features typically associated with hydrophobicity, namely liquid water slip. Further analysis provides details on the molecular mechanism responsible for this surprising result.P rotein folding (1), micelle and cellular membrane formation (2), and frictionless flow of water through carbon nanotube membranes (3-5) are only some manifestations of hydrophobic effects. Flat surfaces are arbitrarily classified as hydrophobic when a water droplet yields a contact angle larger than 90°, hydrophilic otherwise. A now famous 2008 commentary by Granick and Bae (6) initiated a scientific discussion to identify the molecular signature of hydrophobic vs. hydrophilic surfaces. The question is whether or not molecular properties exist for interfacial water molecules that change with the surface "degree of hydrophobicity." Identifying such properties could advance practical applications (e.g., designing self-cleaning surfaces) as well as fundamental scientific endeavors including understanding self-assembly (7).Molecular simulations should allow the discovery of such molecular signatures because they allow a systematic variation of the properties of a surface, as well as of surface-water interactions (8). Although the resultant substrates may not be realistic, the results are useful to interpret nature and to design innovative materials. It has so far been possible to relate some macroscopic observables to the degree of hydrophobicity [i.e., contact angle to adsorption free energy (9)]. Garde and coworkers employed equilibrium molecular dynamics (MD) to determine a number of quantities, including local density, contact angle, and adsorption of small solutes for water near surfaces of varying degrees of hydrophobicity (10). Whereas the local water density provided unsatisfactory characterization, the probability of cavity formation was found to be large near hydrophobic and small near hydrophilic surfaces.The present work focuses on the relation between one important macroscopic signature of hydrophobic surfaces, the hydrodynamic liquid slip, to molecular interfacial water properties. Large liquid slip on hydrophobic surfaces could reduce the drag in vessels navigating the seas, the pressure drop encountered by fluids flowing inside pipes, and even repel ice formation. Liquid slip seems to appear when ...

The flow field resulting from two similar converging-plane jet nozzles was studied using a computational fluid dynamics approach that was validated through experimental data. The case in which the nozzles were near each other (blunt die) and the case in which there was no space between the nozzles (sharp die) were both considered. Such rectangular nozzles are used commercially to produce polymeric fibers in melt-blowing processes. The k-turbulence model and the Reynolds stress model were used. The model parameters were calibrated by using the experimental data; accurate model predictions resulted from this calibration. The flow field downstream from the blunt die was found to exhibit (a) a region in which each jet has its own identity; (b) a merging region, which includes a maximum in turbulence intensity; and (c) a self-similar region. The flow field for the sharp die exhibited only the latter two regions of development. The behavior of alternative die designs, with different jet angles, was also examined. As the jet angle becomes sharper, the mean velocity under the die increases, but at the same time, the turbulence becomes stronger.

A review of the characteristics of hydrophobicity is presented, with the goal of investigating the relationship, if any, between the contact angle (a macroscopically observed property) and the slip length (a microscopic phenomenon). An analysis of simulations, and of their evolution through the years, sheds light on some inherent differences between contact angle and slip length behavior on flat and patterned surfaces. Previous studies lead to the conclusion that epitaxial layering of fluid near the solid is intricately related to the magnitude of fluid slip. Epitaxial layer data help to explain unexpected slip length behavior in relation to the contact angle, and reported inconsistencies between slip length experiments and simulations. Therefore, it seems that solids that can produce favorable epitaxial layering of the fluid will cause larger slip. Dimensional analysis is used to elucidate the contact angle-slip length relationship. Results can be applied to the development of artificial supersolvophobic surfaces that would exhibit predictable fluid slip with important practical applications.

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Sharp dies are often used commercially to produce polymeric fibers in the melt-blowing process. In these sharp dies, the flow field results from two similar converging plane jet nozzles with no space between the nozzles. This study utilizes a computational fluid dynamics approach that is validated through experimental data to investigate the effect of recess or excess (inset or outset) of the die nose on the flow field. The Reynolds Stress Model is used to simulate the turbulence, and the model parameters are calibrated with experimental data. The flow field downstream from the sharp die is found to exhibit (a) a merging region, which includes a maximum in turbulence intensity, and (b) a self-similar region. The behavior of alternative die designs is correlated to the die configuration. The more that the nose piece is recessed, the larger is the mean velocity under the die, but at the same time the turbulence becomes stronger.

Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter epsilonr=epsilonsfepsilonff and the size parameter sigmar=sigmasfsigmaff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25 degrees to 147 degrees on highly hydrophobic surfaces (as epsilonr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilonr and sigmar: increasing logarithmically with decreasing surface energy epsilonr (i.e., more hydrophobic), while decreasing with power law with decreasing size sigmar. The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilonr) produce hydrophobicity, larger sigmar smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100 to 10(5) ms. We demonstrate that fluid-solid interfaces with low epsilonr and high sigmar should be chosen to increase slip and are prime candidates for drag reduction.