Evaporating Process Of Nanofluid Sessile Droplet And Deposition Patterns Of Nanoparticles

Nanofuild is considered as giant potential working medium for the wide use of nanofluid droplet evaporation in coating, painting, and surface pattern structures. It is a universal phenomenon that a liquid droplet evaporates when it is surrounded by its unsaturated vapor. A simple evaporation process actually contains a complex process of heat and mass transfer. The superior properties in thermal conductivity make it have applications in many fields, such as high integration super cooling technology of miniaturization of electronic products. However a series of new problems such as the increase of flow resistance and particle deposition congestion come along with the process of droplet evaporation. Therefore it has important research significance to make clear the mechanism of the nanofluid droplet evaporation and its internal flow regimes.In this paper, we performed a experimental study on evaporation behavior of Al₂O₃-H₂ O nanofluid droplets, including different size categories（10 nm, 50 nm, 100 nm）, different mass concentration（0.05%, 0.2%, 1%）of the nanoparticles evaporating on three different substrates（copper, iron, glass）with varying temperature conditions of 30 ℃, 40 ℃, 50 ℃ and 60 ℃. The change of contact angle and triple line versus time of deionized water droplets and nanofluid droplets with different concentrations during evaporation are analyzed. We also tracked and recorded the profile of droplets during evaporation. It is concluded the influence of nanoparticles on the evaporation regimes. It is also observed by microscope the deposition patterns left on substrate after fully evaporation, so as to induce the inside flow of droplet during evaporation. The main results of this research is stated as following:1. The real particle size distribution of nanofluid is not necessarily the same as its apparent value, there is a certain range of the distribution values. The aggregation of nanoparticles may induce to lager particle group and even to micron order of magnitude. The roughnesses of three kinds of substrates are all of nanometer order of magnitude and the iron substrate is of biggest roughness, copper following by it while the glass substrate is the smallest.2. Droplet evaporation of the experimental results shows that the surface properties such as roughness and hydrophilic are all have effects on droplet evaporation models. In the substrate of iron, which is more rough, the stick-slip motion is more frequent, while in the relatively slippy glass surface, it tends to have a constant- contact line evaporation model. The addition of nanoparticles modified the surface character of substrates so as to verify the equilibrium contact angle and evaporating regimes. Values of contact angle on glass substrates are much lower than that on mental substrates. The influence of concentration on equilibrium contact angle shows completely opposite tendency on substrates of different wetting properties. This may due to the deposition of nanoparticles, resulting in the change of surface properties, therefore to minish the difference of equilibrium contact angle between hydrophobic and hydrophilic surfaces.3. Parameters of substrate temperature, particle diameter, and concentration of nanofluid have certain effects on the deposition patterns of nanoparticles after evaporation. There are roughly four kinds of deposition pattern: coffee ring pattern, concentric rings pattern, uniform pattern, and aggregation structure inside the coffee ring pattern. For nanoparticle diameter of 10 nm, deposition patterns tend to form coffee ring structure. With relative higher concentrations, it is more like concentric rings patterns, and the proportion of deposition in the central part becomes bigger under. For nanoparticle diameter of 50 nm, it is more likely to form uniform patterns. With higher substrate temperature, more particles would deposit on the edge, owing to higher evaporation rate inducing movement of particles towards the triple line. For 100 nm,a combined structure will be formed. Most of the patterns could be a described as aggregation structure inside the coffee ring. The wide range of 100 nm particle distribution may accounted for the structure that the smaller particles go outward toward the contact line while the bigger particle groups tend to get deposit at the early stage of evaporation. The Maragoni number we calculated from the experiment data, has a good explanation for the deposition patterns from the nanofluid evaporation and the inner flow regimes.