Influence Of Thermal Properties Of Substrate On The Evaporation Of Nanofluid Sessile Droplets

Owing to its good thermal conductivity, nanofluid is a potential novel workingmedium. The thermal properties of nanofluid are different from base fluid because of theadding of nanoparticles. Sessile droplet evaporation has a significant influence on theelectronics cooling, fire suppression, pesticide spray, ink-jet printing and even the medicaldiagnosis. The present paper experimentally measured the saturated vapor pressure,viscosity and surface tension of Al2O3-H2O nanofluid in different temperature range byusing the saturated vapor pressure setup, viscometer and surface tension PocketDyne. Theevaporation of Al2O3-H2O nanofluid sessile droplets on three different substrates (copper,aluminum and iron) also studied. DSA30was used to measure the contact angle, contactdiameter and height of the droplets. The results showed that:(1) In the temperature range of30℃-50℃, the saturated vapor pressure of nanofluid is lessthan that of base fluid which is deionized water. The nanofluid structure changes because ofnanoparticles. When the particle size is10nm or20nm, the intermediate physical statecalled the liquid-like solid layers exists in the nanofluid. When the particle size is50nm or100nm, the intermediate physical state becomes to be the solid-like liquid layers. The twolayered structures may resist the water molecules escaping from the vapor/liquid interface,decreasing the number of free water molecules.(2) In the temperature range of18℃-40℃, the viscosity of nanofluid decrease with theincrease of temperature and increase with the increase of volume fraction. Relativeviscosity (viscosity ratio of nanofluid and base fluid) elevates with the increase of volumefraction.1.5vol%is the critical concentration. When the volume fraction is less than1.5vol%, the increase curve of relative viscosity becomes flat. When the volume fraction ismore than1.5vol%, the increase curve of relative viscosity becomes steeper. The little theparticle size is, the more significant the influence of nanoparticle on base fluid will be.Particles with small size are easier to aggregate, which may affect the viscosity ofnanofluid.(3) In the temperature range of18℃-30℃, the surface tension of nanofluid decrease withthe elevate of temperature and decrease with the increase of volume fraction. The reason isthat the nanoparticles act as surfactant on the vapor/liquid interface, which may decreasethe surface tension. Since the results are completely opposite to that of Godson, it can bededuced that there may exist a critical value, which decides whether the surface tension ofnanofluid increase with the increase of volume fraction. it needs further investigation. Theeffect of particles on the surface tension of base fluid depends on temperature and particle diameter. With the constant temperature, there is corresponding critical concentration. Onlyif reaching to the value, the adding of nanoparticles can increase the surface of base fluid.With constant particle diameter, the critical concentration is inversely proportional to thetemperature. With constant temperature, the critical concentration is inversely proportionalto the particle diameter.(4) During the evaporation, the thermal properties affect the variation of the contactdiameter, contact angle, evaporation time and particle deposition patterns of sessile droplets.The thicker the substrate is, the more flat the decrease curve of contact angle is. Decreasingthe thermal conductivity or increasing the thickness of substare can lead to the decrease ofthe evaporation time.All in all, the particles change the propeties of Al2O3-H2O nanofluid. And the thermalproperties of the substrates affect the evaporation of sessile droplets. The thickness affectsthe contact angle varation, and the thermal resistance changes the time of constant contactline stage and the whole evaporation. The results are related to the cooling effect of thedroplet.