Sustainability of apparent slip in micro-channel flows

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.In the present work, we experimentally study the flow of water over textured hydrophobic surfaces in a micro-channel. Shear stress measurements are done along with direct visualization of trapped air pockets on the hydrophobic surface. The trapped air pockets on such surfaces are known to be responsible for apparent slip at these surfaces and hence in significant drag reduction. In typical circumstances, the apparent slip reduces over time as seen, for example, from our shear stress measurements. This implies that the drag reduction will not be sustained. We have performed extensive visualizations of the trapped air pockets while varying flow parameters like the flow rate and the pressure. We present here direct visualizations that show that under some conditions, the air pockets can grow with time. The variation of the air pocket size with time is found to change qualitatively and quantitatively as the flow rate is varied. These measured changes in the air pocket size with time have a direct bearing on the sustainability of apparent slip in micro-channel flows

Time dependent superhydrophobicity of drag reducing surfaces

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Air can be trapped on the crevices of specially textured hydrophobic surfaces immersed in water. This heterogenous state of wetting in which the water is in contact with both the solid surface and the entrapped air is not stable. Diffusion of air into the surrounding water leads to gradual reduction in the size and numbers of the air bubbles. The sustainability of the entrapped air on such surfaces is important for many underwater applications in which the surfaces have to remain submersed for longer time periods. In this paper we explore the suitability of different classes of surface textures towards the drag reduction application by evaluating the time required for the disappearance of the air bubbles under hydrostatic conditions. Different repetitive textures consisting of holes, pillars and ridges of different sizes have been generated in silicon, aluminium and brass by isotropic etching, wire EDM and chemical etching respectively. These surfaces were rendered hydrophobic with self-assembled layer of fluorooctyl trichlorosilane for silicon and aluminium surfaces and 1-dodecanethiol for brass surfaces. Using total internal reflection the air bubbles are visualized with the help of a microscope and time lapse photography. Irrespective of the texture, both the size and the number of air pockets were found to decrease with time gradually and eventually disappear. In an attempt to reverse the diffusion we explore the possibility of using electrolysis to generate gases at the textured surfaces. The gas bubbles are nucleated everywhere on the surface and as they grow they coalesce with each other and get pinned at the texture edges

Formation of a hard surface layer during drying of a heated porous media

We report surface hardening or crust formation, unlike caking, during drying when a confined porous medium was heated from above using IR radiation. These crusts have higher strength than their closest counterparts such as sandcastles and mud-peels which essentially are clusters of partially wet porous medium. Observed higher strength of the crusts is mostly due to surface tension between the solid particles which are connected by liquid bridges (connate water). Qualitative (FTIR) and quantitative (TGA) measurements confirm the presence of trapped water within the crust. Amount of the trapped water was ~1.5% (this is about 10 times higher than in the samples with caking) which was confirmed using SEM images. Further, in the fixed particle sizes case, the crust thickness varied slightly (10-20 particle diameters only for cases with external heating) while with the natural sand whole porous column was crusted; surprisingly, crust was also found with the hydrophobic glass beads. Fluorescein dye visualization technique was used to determine the crust thickness. We give a power law relation between the crust thickness and the incident heat flux for various particle sizes. The strength of the crust decreases drastically with increasing hydrophilic spheres diameter while it increases with higher surface temperature.Comment: 17 pages, 7 figures, 1 table Information regarding 'Supplementary Information File' is mentioned in the main tex

Depth-sensing indentation tests in studying plastic instabilities

This review surveys the phenomenon of plastic instabilities occurring in depth-sensing indentation measurements. Investigations presented focus on the characterization of Portevin-Le Chatelier type instabilities observed in different metal alloys during indentation. The effect of some important factors such as solute concentration, the formation of Guinier-Preston zones, and grain size and orientation are described and discussed. The phenomenon of plastic instabilities as serrated flow recently observed in bulk metallic glasses is also briefly reviewed

Springing the trap

Charles Darwin is known the world over as the founder (along with A R Wallace) of modern evolutionary biology. But he wrote a great many books besides The Origin of Species, and all of them illuminate the astonishing ways in which evolution works. In one of those books, Insectivorous Plants, Darwin examined plants that ate animals – in contrast to the usual situation, which is the other way round. Carnivorous plants seem to violate another of nature’s rules: some of them possess the property of thigmonasty or touch-induced movement. Because they display the behaviour without nerves or muscles (though they are not unique in this; see Bonner 1994), carnivorous and sensitive plants, like the familiar Mimosa pudica (touch-me-not), raise the question of where to draw the boundary between plants and animals

Effect of wettability and surface roughness on ice-adhesion strength of hydrophilic, hydrophobic and superhydrophobic surfaces

The anti-icing properties of hydrophilic, hydrophobic and superhydrophobic surfaces/coatings were evaluated using a custom-built apparatus based on zero-degree cone test method. The ice-adhesion reduction factor (ARF) of these coatings has been evaluated using bare aluminium alloy as a reference. The wettability of the surfaces was evaluated by measuring water contact angle (WCA) and sliding angle. It was found that the ice-adhesion strength on silicone based hydrophobic surfaces was ~ 43 times lower than compared to bare polished aluminium alloy indicating excellent anti-icing property of these coatings. Superhydrophobic coatings displayed poor anti-icing property in spite of their high water repellence. Field Emission Scanning Electron Microscope reveal that Silicone based hydrophobic coatings exhibited smooth surface whereas the superhydrophobic coatings had a rough surface consisting of microscale bumps and protrusions superimposed with nanospheres. Both surface roughness and surface energy play a major role on the ice-adhesion strength of the coatings. The 3D surface roughness profiles of the coatings also indicated the same trend of roughness. An attempt is made to correlate the observed ice-adhesion strength of different surfaces with their wettability and surface roughness. It was concluded that smooth surface with low surface energy are responsible for low ice-adhesion strength