Surface texturing at the micro/nanolevel allows air to be trapped in sufficiently small cavities, thereby reducing the flow resistance over the surface in the laminar regime. The nature of the liquid–gas meniscus plays an important role in defining the boundary condition and it depends on the flow conditions and geometrical properties of textures. In the present work, we employ the unsteady volume of fluid model to investigate the behavior of the liquid–gas meniscus for ridges arranged normal to the flow direction to substantiate the frictional resistance of flow in a microchannel. It is found that the assumption of ‘zero shear stress’ at the liquid–gas interface grossly overpredicts the effective slip length with meniscus curvature and local partial slip length playing the dominant role. Numerical simulations performed in the laminar regime (20  <  Re  <  120) over single layered ridges normal to the flow direction revealed the effect of texture geometry on the reduction in pressure drop. In single layered structures, lotus-like geometries exhibited a greater reduction in drag (more than 30%) when compared to all other texture geometries. It is recognized that the flow experiences expansion and contraction cycles as it flows over the transverse ridges increasing the frictional resistance. Our findings will help to modify the boundary condition at the liquid–gas meniscus for accurate modeling in the laminar regime and to optimize the texture geometry to improve drag reduction.

There are several examples in nature where the biological surfaces exhibit unique functional response, such as velcro, fish scale and lotus leaves. The texture on lotus leaf exhibits super-hydrophobicity and self cleaning properties. Lotus leaf has hemispherical protrusions of 20–30 μm in diameters which are randomly distributed over the surface. This work is focused on creating similar textured surfaces on Ti6Al4V rods via a vibration assisted reverse micro Electrical Discharge Machining (R-MEDM) process. Textured surfaces containing micropillars of 40–50 μm in diameter spaced at 35 μm have been created during the process. These textured surfaces are expected to exhibit hydrophobicity and hemocompatibility. To experimentally characterize the process, a full factorial design of experiments has been conducted to analyze the effects of voltage, capacitance, amplitude and frequency of the anode (plate electrode) vibrations on the erosion rate and process stability. The process stability is expressed in terms of the percentages of the normal, open circuit and the short circuit durations in the voltage-current (VI) signature obtained during the process. It has been observed that the normal discharge durations increase with an increase in the amplitude and the frequency of the vibrations. Fabricated texture exhibits hydrophobicity and the measured contact angles in a sessile drop test with water varied between 110 and 115°. Also, the textured surface was subjected to hemotoxicity tests which yielded positive results. Based on these results, it can be seen that the machined textured surface are hydrophobic and biocompatible in nature which could potentially find applications in cardiovascular biomedical implants. In addition, this process has been used to create hierarchical structures comprising of primary and a secondary structure over it to mimic the hierarchical structures found on lotus leaves.

High aspect ratio arrayed micro-structures and textured surfaces are required in diversified applications such as electrical contacts, printing heads, electrodes for micro-batteries, injection nozzles, nano-material delivery systems, biomedical implants, and hydrophobic surfaces. Reverse micro-electrical discharge machining (R-MEDM) process has a capability to fabricate such arrayed features on a variety of workpiece materials irrespective of their mechanical properties. R-MEDM is a variant of micro-electrical discharge machining (MEDM) process, key difference being, extruded arrayed features are fabricated in the R-MEDM process against the micro-cavities that are machined in MEDM. This article highlights the recent advances in process characterization and modelling of mechanics of the R-MEDM process. The focus of discussion is on comparing the process with the other micromachining processes presently available for the fabrication of arrayed micro-features. In addition, R-MEDM process characteristics in the fabrication of arrayed features on ‘easy’ and ‘difficult’ to erode materials are presented. It is understood that R-MEDM has comparable or in some cases better performance in the fabrication of arrayed features than the processes like micro-milling, micro-wire EDM, micro-wire electrical discharge grinding (EDG) and block EDG.

The machining accuracy of Electrochemical Machining (ECM) is affected by the temperature and bubbles. The effect of bubbles in the simulation can be considered by first calculating the volume fraction of gas distribution (using bubbly flow approximation in this study) and then approximating the effective conductivity in the inter-electrode gap by using Bruggemann equation. Since, Bruggemann equation is independent of bubble diameter and distribution, its dependency on that was investigated in this study. Moreover, the effect of the bubbles generated on the machining accuracy of the axi-symmetric work piece with different flow rates was also investigated.