Comment on ``Large Slip of Aqueous Liquid Flow over a Nanoengineered Superhydrophobic Surface'' by C-H Choi and C Kim

We present a comprehensive study of water drops sliding down chemically heterogeneous surfaces formed by a periodic pattern of alternating hydrophobic and hydrophilic stripes. Drops are found to undergo a stick-slip motion whose average speed is an order of magnitude smaller than that measured on a homogeneous surface having the same static contact angle. This motion is the result of the periodic deformations of the drop interface when crossing the stripes. Numerical simulati...

It is commonly assumed that fluid cannot slip along a solid surface. The experimental evidence generally supports this assumption. We demonstrate that when the change of the relative velocity of a fluid and a solid wall is sufficiently rapid, the slip does occur; the fluid is unable to adjust if acceleration is large enough, and it slips. We use droplet impact on a moving surface to demonstrate and estimate the slip length. We also estimate fluid acceleration, which is requir...

Stability of lubricating fluid infused slippery surfaces is a concern for scientists and engineers and attempts are being made for its improvement. Lubricating oil coated slippery surface for aqueous drops is one of the important candidates in this class and their stability needs be improved to make them useful for practical applications. Cloaking of water drops with thin lubricant layer results in the loss of lubricant leading to deterioration of slippery behavior. Surface r...

Thousands of plant and animal species have been observed to have superhydrophobic surfaces that lead to various novel behaviors [1-5]. These observations have inspired attempts to create artificial superhydrophobic surfaces, given such surfaces have multitudinous applications [6-13]. Superhydrophobicity is an enhanced effect of surface roughness and there are known relationships that correlate surface roughness and superhydrophobicity, based on the underlying physics. However...

We analyze theoretically a high-speed drainage of liquid films squeezed between a hydrophilic sphere and a textured super-hydrophobic plane, that contains trapped gas bubbles. A super-hydrophobic wall is characterized by parameters $L$ (texture characteristic length), $b_1$ and $b_2$ (local slip lengths at solid and gas areas), and $\phi_1$ and $\phi_2$ (fractions of solid and gas areas). Hydrodynamic properties of the plane are fully expressed in terms of the effective slip-...

The hydrodynamic slippage at a solid-liquid interface is currently at the center of our understanding of fluid mechanics. For hundreds of years this science has relied upon no-slip boundary conditions at the solid-liquid interface that has been applied successfully to model many macroscopic experiments, and the state of this interface has played a minor role in determining the flow. However, the problem is not that simple and has been revisited recently. Due to the change in ...

On microstructured hydrophobic surfaces, geometrical patterns may lead to the appearance of a superhydrophobic state, where gas bubbles at the surface can have a strong impact on the fluid flow along such surfaces. In particular, they can strongly influence a detected slip at the surface. We present two-phase lattice Boltzmann simulations of a flow over structured surfaces with attached gas bubbles and demonstrate how the detected slip depends on the pattern geometry, the bul...

The transport of small quantities of liquid on a solid surface is inhibited by the resistance to motion caused by the contact between the liquid and the solid. To overcome such resistance, motion can be externally driven through gradients in electric fields, but these all inconveniently involve the input of external energy. Alternatively, gradients in physical shape and wettability - the conical shape of cactus spines to create self-propelled motion. However, such self-propel...

We discuss how the wettability of solid walls impacts electrokinetic properties, from large systems to a nanoscale. We show in particular how could the hydrophobic slippage, coupled to confinement effects, be exploited to induce novel electrokinetic properties, such as a salt-dependent giant amplification of zeta potential and conductivity, and a much more efficient energy conversion. However, the impact of slippage is dramatically reduced if some surface charges migrate alon...

We show that an electro-osmotic flow near the slippery hydrophobic surface depends strongly on the mobility of surface charges, which are balanced by counter-ions of the electrostatic diffuse layer. For a hydrophobic surface with immobile charges the fluid transport is considerably amplified by the existence of a hydrodynamic slippage. In contrast, near the hydrophobic surface with mobile adsorbed charges it is also controlled by an additional electric force, which increases ...