Mohamed A. Samaha

Modeling drag reduction and meniscus stability of superhydrophobic surfaces comprised of random roughness

Previous studies dedicated to modeling drag reduction and stability of the air-water interface on superhydrophobic surfaces were conducted for microfabricated coatings produced by placing hydrophobic microposts/microridges arranged on a flat surface in aligned or staggered configurations. In this paper, we model the performance of superhydrophobic surfaces comprised of randomly distributed roughness (e.g., particles or microposts) that resembles natural superhydrophobic surfaces, or those produced via random deposition of hydrophobic particles. Such fabrication method is far less expensive than microfabrication, making the technology more practical for large submerged bodies such as submarines and ships. The present numerical simulations are aimed at improving our understanding of the drag reduction effect and the stability of the air-water interface in terms of the microstructure parameters. For comparison and validation, we have also simulated the flow over superhydrophobic surfaces made up of aligned o...

Influence of flow on longevity of superhydrophobic coatings.

Previous studies have demonstrated the capability of superhydrophobic surfaces to produce slip flow and drag reduction, which properties hold considerable promise for a broad range of applications. However, in order to implement such surfaces for practical utilizations, environmental factors such as water movement over the surface must be observed and understood. In this work, experiments were carried out to present a proof-of-concept study on the impact of flow on longevity of polystyrene fibrous coatings. The time-dependent hydrophobicity of a submerged coating in a pressure vessel was determined while exposing the coating to a rudimentary wall-jet flow. Rheological studies were also performed to determine the effect of the flow on drag reduction. The results show that the longevity of the surface deteriorates by increasing the flow rate. The flow appears to enhance the dissolution of air into water, which leads to a loss of drag reduction.

In situ, noninvasive characterization of superhydrophobic coatings.

Light scattering was used to measure the time-dependent loss of air entrapped within a submerged microporous hydrophobic surface subjected to different environmental conditions. The loss of trapped air resulted in a measurable decrease in surface reflectivity and the kinetics of the process was determined in real time and compared to surface properties, such as porosity and morphology. The light-scattering results were compared with measurements of skin-friction drag, static contact angle, and contact-angle hysteresis. The in situ, noninvasive optical technique was shown to correlate well with the more conventional methods for quantifying surface hydrophobicity, such as flow slip and contact angle.

Sustainability of superhydrophobicity under pressure

Prior studies have demonstrated that superhydrophobicity of submerged surfaces is influenced by hydrostatic pressure and other environmental effects. Sustainability of a superhydrophobic surface could be characterized by both how long it maintains the trapped air in its surface pores, so-called “longevity,” and the pressure beyond which it undergoes a global wetting transition, so-called “terminal pressure.” In this work, we investigate the effects of pressure on the performance of electrospun polystyrene fibrous coatings. The time-dependent hydrophobicity of the submerged coating in a pressure vessel is optically measured under elevated pressures. Rheological studies are also performed to determine the effects of pressure on drag reduction and slip length. The measurements indicate that surface longevity exponentially decays with increasing pressure in perfect agreement with the studies reported in the literature at lower pressures. It is found, however, that fibrous coatings could resist hydrostatic pressures significantly higher than those of previously reported surfaces. Our observations indicate that superhydrophobic fibrous coatings could potentially be used for underwater applications.

Novel method to characterize superhydrophobic coatings.

Superhydrophobic coatings possess a strong water-repellent characteristic, which, among several other potential applications, enhances the mobility of water droplets over the surface. The coating traps air within its micropores, such that a submerged moving body experiences shear-free and no-slip regions over, respectively, the air pockets and the solid surface. This, in turn, may lead to significant skin-friction reduction. The coating maintains its superhydrophobicity as long as the air remains entrapped. It is therefore of great interest to precisely measure the amount of trapped air, which is particularly difficult to estimate for coatings with disordered microstructures. A novel method to measure the effective thickness and gas volume fraction of superhydrophobic coatings with either ordered or random microroughness is advanced. The technique is applied to both aerogel and electrospun fibrous coatings. The experiments utilize a sensitive weighing scale (down to 10(-4) gm) and height gauge (down to 10 μm) to determine the buoyancy force on an immersed, coated glass-slide substrate. The measured force is used to calculate the volume fraction of entrapped air. The coatings effective thickness also follows from the same calculations. The sensitivity of our particular scale enables the measuring of thicknesses down to 3 μm, which is not readily possible with conventional thickness gauges. Smaller thicknesses could be measured using more sensitive scales.

Errors in parallel-plate and cone-plate rheometer measurements due to sample underfill

The effect of sample underfill on parallel-plate and cone-plate rheometers is examined. Sample underfill can be caused by incomplete filling of a sample or loss of fluid during a test by, for example, evaporation. It is shown that even a small degree of sample underfill can lead to significant errors in measuring viscosity. A method is proposed to reduce these errors by directly monitoring the sample radius over the full course of the test. It is shown that the accuracy of the rheometer even while testing simple fluids like water is greatly improved.

Stratified thin-film flow in a rheometer

When two immiscible layered fluids are present in a rheometer, interfacial distortions driven by the centripetal pressure gradient can modify torque measurements and induce dewetting. In particular, we examine the steady-state interface shape of a thin film coating a stationary substrate beneath a second immiscible fluid that is driven by a rotating parallel-plate or cone. An asymptotic analysis of the interfacial distortion for the parallel-plate flow is compared with numerical solutions for both the parallel-plate and cone and plate configurations. We develop asymptotic criteria for dewetting of the thin film as a function of fluid and flow properties, and show that significant interfacial distortion and dewetting can occur due to secondary flow effects even at low Reynolds numbers. The distortion of the interface can result in increased or decreased torque measurements depending on the viscosity and density ratios between the two fluid layers. We relate these effects to recent experimental studies on l...

Convective Mass Transfer From Submerged Superhydrophobic Surfaces

Longevity of entrapped air is an outstanding problem for using superhydrophobic coatings in submersible applications. Under pressure and flowing water, the air micropockets eventually dissolve into the ambient water or burst and diminish. Herein, we analyze from first principles a simple mass transfer problem. We introduce an effective slip to a Blasius boundary layer, and solve the hydrodynamic equations. A slowly evolving, non-similar solution is found. We then introduce the hydrodynamic solution to the two-dimensional problem of alternating solid-water and air-water interfaces to determine the convective mass transfer of airs dissolution into water. This situation simulates spanwise microridges, which is one of the geometries used for producing superhydrophobic surfaces. The mass-transfer problem has no similarity solution but is solvable using approximate integral methods. A mass-transfer solution is achieved as a function of the surface geometry (or gas area fraction), Reynolds number, and Schmidt n...

Convective Mass Transfer From Submerged Superhydrophobic Surfaces: Turbulent Flow

Superhydrophobic surfaces have received considerable attention in recent years. The surface has a strong water-repellent characteristic that could produce slip flow and drag reduction. The coating traps air within its micropores, such that a submerged moving body experiences shear-free and no-slip regions over, respectively, the air pockets and the solid surface. This, in turn, holds promise for a broad range of applications. Longevity of the entrapped air is an outstanding problem for these coatings. Under pressure and flowing water, the air micropockets eventually dissolve into the ambient water or burst and diminish. Herein, we analyze from first principles an air mass transfer problem. Using integral methods, we extend our prior laminar flow solution to turbulent flows. We introduce an effective slip to the turbulent boundary layer characterized by a modified 1/7-power law velocity profile. We then introduce the hydrodynamic solution to the two-dimensional problem of alternating solid-water and air-wa...

Fabrication and Characterization of Low-Cost Superhydrophobic Coatings

Most previous studies dedicated to modeling and testing drag reduction and stability of the air{water interface on submerged superhydrophobic surfaces were conducted using microfabricated coatings produced by placing hydrophobic microposts/microridges arranged in aligned or staggered con gurations on a small-scale at surface. In this paper, we numerically model and experimentally characterize the performance of superhydrophobic surfaces made either using AC-electrospun bers or random deposition of hydrophobic particles. Such fabrication methods are far less expensive than microfabrication, bringing the technology closer to large-scale submerged bodies such as submarines and ships.