Spreading and Wetting Behaviour of Trisiloxanes

Wetting and spreading processes which involve surfactant solutions are widely used in numerous industrial and practical applications nowadays.The performance of different non-ionic surfactants may vary significantly and so far superspreader solutions show the most promising spreading ability.The addition of trisiloxane surfactants to water was proven to enhance wetting,even on hydrophobic surfaces,on which conventional surfactants seem to have little or no effect.Although these extraordinary surfactants have been extensively studied over recent years,complete understanding of their underlying mechanisms and a suitable mathematical model are still lacking.Here we present a possible explanation for the impressive performance of trisiloxane,which is compared to wetting enhancement of a conventional surfactant.Additionally,we will explain why the hydrophobicity of the surface is a crucial factor for the spreading phenomenon.Light will be also shed on the effect of the pH of the solution to which surfactants are added.Finally,we will investigate long-term effects of the water environment on trisiloxane wetting ability and discuss if ageing may significantly affect their performance.

A Study of the Velocity Field during Evaporation of Sessile Water and Water/Ethanol Drops

Many studies have investigated evaporation of sessile drops in an attempt to understand the effect of wetting on theevaporation process.Recently interest has also increased in the deposition of particles from such drops,with evaporative massflux being deemed to be responsible for ring-like deposits,and counteraction of the mass flux by Marangoni convection explainingmore uniform deposition patterns.Understanding of such deposition processes is important in biological applications,such as the Litos test-system endorsed by the Russian Ministry of Health for diagnosis of urolithiasis and the evaporation ofcolloidal drops for depositing and organizing proteins and DNA.In most cases where deposition from evaporating drops hasbeen studied,velocity information is inferred from the final deposition pattern or from mathematical modeling based on simplifiedmodels of the physics of the evaporation process.In this study we have directly measured the flow velocities in the baseof sessile drops,using micro particle image velocimetry,viewing the drop from below,through the cover slide.For water drops,a radial pattern of flow was observed with a maximum velocity close to but not at the pinned outer edge.For ‘azeotropic’ethanol/water mixtures,the velocity field is more chaotic to begin with,passing through a phase involving three or four recirculationcells and finally having the same radial pattern as for water drops.

Biomimetic Approaches to Functional Surfaces,Surface Wetting and Fluids Drag Reduction

In the natural world,plants and animals haveevolved over time to best adapt to the environment.Theyinteract very effectively with the surrounding environmentby exchanging energies and mass flow across theircuticles of specific micro structures and functions toachieve perfect energy balance.Such different functionsmay include the limitation of uncontrolled loss of water,protection from solar radiation,micro effect of inducedturbulence on flow drag reduction,defence againstpathogens,changing surface wettability and hydropho-

Geometrical Deposits on Microstructured Surfaces

Research into evaporating droplets on patterned surfaces has grown exponentially,since the capacity to control droplet morphology has proven to have significant technological utility in emerging areas of fundamental research and industrial applications.Here,we incorporate two interest domains-complex wetting patterns of droplets on structured surfaces and the ubiquitous coffee-ring phenomenon of nanofluids containing dispersed aluminium oxide particles.We lay out the surface design criteria by quantifying the effect of pillar density and shape on the wetting footprint of droplets,yielding complex polygon droplet geometries.Our work is not constrained to pure liquids only,as we delve into the shape selection of particle-laden droplets of different concentrations.We visualise the deposition patterns through microscopy on surfaces exhibiting different features and further establish the ordering of particles on microscale surface asperities.At a high nanofluid concentration,we observe intriguing self-assembly of particles into highly ordered intricate structures.The collective findings of this work have the potential to enhance many industrial technologies,particularly attractive for high performance optical and electrical devices.

Preface-Selected Papers from UKHTC2019

We are very pleased to introduce the following selected and peer reviewed papers from the 16th UK National Heat Transfer Conference(UKHTC2019)to Journal of Bionic Engineering(JBE),which is related to nature inspired research works on bubbles and drops patterns as well as wettings and heat transfer.The UKHTC2019 conference brought together scientists and engineers,working on a wide range of topics of thermal science and engineering to exchange information on the state-of-the-art and recent developments in areas related to the Conference themes,and to encourage fundamental studies of heat and mass transfer as well as the practical and efficient use of thermal energy.Nature inspired heat transfer is one of the conference themes and the selected papers included for JIBE focus on nature inspired solutions of lotus leaf efect,plant capillary effect,and Marangoni effect,etc.The papers were orally presented at UKHTC2019,Notingham,UK,during the period of 8-10 September 2019.