Research into extreme water-repellent surfaces began many decades ago, although it was only relatively recently that the term superhydrophobicity appeared in literature Here we review the work on the preparation of superhydrophobic surfaces, with focus on the different techniques used and how they have developed over the years, with particular focus on the last two years We discuss the origins of water-repellent surfaces, examining how size and shape of surface features are used to control surface characteristics, in particular how techniques have progressed to form multi-scaled roughness to mimic the lotus leaf effect There are notable differences in the terminology used to describe the varying properties of water-repellent surfaces, so we suggest some key definitions

Progess in superhydrophobic surface development.

The paper reviews progress in dropwise condensation research from 1930 to the present. Particular attention is given to heat transfer measurements, theory, transition and effects of surface material. Although it has been known since the 1930s that heat transfer coefficients for dropwise condensation of steam are much higher than those for film condensation, there were, until the 1960s, wide discrepancies between the results of different investigators. Subsequently, more accurate measurements have shown good consistency and the mechanism and theory of the dropwise condensation have become better understood. There has been considerable controversy over the magnitude of the so-called ‘constriction resistance’ and the effect of the surface thermal conductivity on the heat transfer coefficient. The balance of evidence suggests that this is only significant at very low heat fluxes and for very small condensing surfaces. Measurements have also been made with sufficiently high cooling intensities to cover...

Dropwise condensation theory and experiment: A review

The regulation of water content in polymeric membranes is important in a number of applications, such as reverse electrodialysis and proton-exchange fuel-cell membranes. External thermal and water management systems add both mass and size to systems, and so intrinsic mechanisms of retaining water and maintaining ionic transport in such membranes are particularly important for applications where small system size is important. For example, in proton-exchange membrane fuel cells, where water retention in the membrane is crucial for efficient transport of hydrated ions, by operating the cells at higher temperatures without external humidification, the membrane is self-humidified with water generated by electrochemical reactions. Here we report an alternative solution that does not rely on external regulation of water supply or high temperatures. Water content in hydrocarbon polymer membranes is regulated through nanometre-scale cracks ('nanocracks') in a hydrophobic surface coating. These cracks work as nanoscale valves to retard water desorption and to maintain ion conductivity in the membrane on dehumidification. Hydrocarbon fuel-cell membranes with surface nanocrack coatings operated at intermediate temperatures show improved electrochemical performance, and coated reverse-electrodialysis membranes show enhanced ionic selectivity with low bulk resistance.

Nanocrack-regulated self-humidifying membranes

Condensation heat transfer, both inside and outside horizontal tubes, plays a key role in refrigeration, air conditioning and heat pump applications. In the recent years the science of condensation heat transfer has been severely challenged by the adoption of substitute working fluids and new enhanced surfaces for heat exchangers. Well-known and widely established semiempirical correlations to predict heat transfer during condensation may show to be quite inaccurate in some new applications, and consequently a renewed effort is now being dedicated to the characterisation of flow conditions and associated predictive procedures for heat transfer and pressure drop of condensing vapours, even in the form of zeotropic mixtures. This paper critically reviews the most recent results appeared in the open literature and pertinent to thermal design of condensers for the air conditioning and refrigeration industry; both in-tube and bundle condensation are considered, related to the use of plain and enhanced surfaces.

Condensation inside and outside smooth and enhanced tubes: a review of recent research

https://hal.archives-ouvertes.fr/hal-00498952/document

A general review of the Wilson plot method and its modifications to determine convection coefficients in heat exchange devices

Digital computer simulation of dropwise condensation from equilibrium droplet to detectable size

Application of dropwise condensation to utility turbine condensers is investigated by comparing the thermal performance of dropwise and filmwise bundles at industrially relevant conditions. Steam and steam-air mixtures were condensed on bundles of in-line, titanium tubes. The row-by-row heat transfer coefficients are presented against bundle position. They show the expected behavior for filmwise condensation but demonstrate a different one for dropwise. In air-free steam, the dropwise heat transfer coefficients are much larger and do not vary significantly with bundle position. In air-steam mixtures the dropwise values decrease similarly to their filmwise equivalents. The findings are in accord with those found for other geometries. The findings indicate that significant reductions in condenser size can be obtained if permanent dropwise condensation can be produced at industrially relevant conditions.

Dropwise Condensation of Steam on a Small Tube Bundle at Turbine Condenser Conditions

Pressure drop and heat transfer coefficient measurements have been made in a 241 tube bundle kettle reboiler thin slice rig, boiling pentane at atmospheric pressure. The effects of liquid/vapour separation problems in the shell are described and discussed. A conventional 1-D recirculation model predicted the data taken at a uniform heat flux of l0 kW m −2 . Another recirculation model was developed and compared to the data at 50 kW m −2 where the flow in the bundle was markedly two dimensional. This model matched the pressure drop data in the bundle apart from that in the outside columns which were judged to have been affected by adjacent rapidly rising 2-phase flow in the shell. The model was adjusted to eliminate this defect and making use of two flow boiling models reproduced the heat transfer data mainly within experimental error. The consequence of these findings for the application of data from thin sliced rigs to design of full scale kettle reboilers and horizontal recirculation shellside evaporators is discussed.

Heat Transfer Coefficient Distributions in an Experimental Kettle Reboiler Thin Slice

Thin hydrophobic polymer films produced by radio frequency (rf) plasma decomposition of hexamethyldisiloxane (HMDSO) have been produced on flat metal (titanium, stainless steel, and copper/nickel) and silicon substrates. By using a statistical design methodology, the deposition conditions have been assessed for their control of the deposition rate, refractive index, water droplet contact angle, and longevity of dropwise condensation of steam. The films showing the best performance were those deposited at the slower rates, using low rf power density and high monomer flow rates, and having the lower refractive indices. There is some dependence on substrate material, with CuNi behaving worst, and titanium providing the best results of the three metals studied. This may be connected with the stability of an interfacial oxide layer. The steam immersion tests have already exceeded 7500 hours of continuous condensation, and many of the films are still producing excellent dropwise condensation. The reasons for these effects of preparation conditions and substrate material on performance are discussed with reference to the plasma chemical process.

Plasma Polymer Films for Dropwise Condensation of Steam

Thin-films have been deposited onto silicon and metal substrates by plasma polymerization of vinyltrimethylsilane (VTMS), to provide hydrophobic coatings for dropwise condensation of steam. The deposition conditions have been sampled by a set of statistically designed experiments, and the films' thicknesses, water droplet contact angles, elemental compositions (by XPS), and –CHx contents have been measured. There is a strong correlation between these film properties and their deposition conditions. The films that continue to provide successful long-term dropwise condensation had been deposited at a low rate onto heated substrates, such that they have high water contact angle and high –CHx content. This required a plasma that provided a low electron energy input per precursor species collision.

B. M. Burnside

Papers

Digital computer simulation of dropwise condensation from equilibrium droplet to detectable size

Dropwise Condensation of Steam on a Small Tube Bundle at Turbine Condenser Conditions

Heat Transfer Coefficient Distributions in an Experimental Kettle Reboiler Thin Slice

Plasma Polymer Films for Dropwise Condensation of Steam

Hydrophobic Coatings from Plasma Polymerized Vinyltrimethylsilane