Florian Carle

Experimental evidence of the atmospheric convective transport contribution to sessile droplet evaporation

We investigate the contribution of the natural convective transport in the vapor phase on the evaporation rate of an evaporating sessile droplet. When comparing the experimental data with the quasi-steady diffusion-controlled evaporation model, an increasing deviation with substrate temperature that was attributed to the effect of the natural convection on the vapor field has been recently highlighted. To validate this analysis, we present experimental results obtained with two gravity levels: 1 g and μg. The contribution of the natural convection is analyzed with the Grashof number, and an empirical model is developed combining diffusive and convective transport.

How Surface Functional Groups Influence Fracturation in Nanofluid Droplet Dry-Outs

In this study of drying water-based nanofluid droplets, we report the influence of surface functional groups and substrate surface energies on crack formation and dry-out shape. These two key parameters are investigated by allowing nanofluids with several functional groups grafted on polystyrene nanoparticle surfaces to dry on various substrates. These experiments result in a variety of regular crack patterns with identical nanoparticle diameter, material, concentration, and drying conditions. We demonstrate that, despite the various patterns observed, the crack spacing/deposit height ratio is constant for similar substrate surface energies and linearly increases with this parameter. Moreover, this study shows that the crack shape is strongly influenced by surface functional groups as a result of particle interactions (depending on the particle surface potentials) and compaction during solvent evaporation.

Boundary conditions for a one-sided numerical model of evaporative instabilities in sessile drops of ethanol on heated substrates

The work is focused on obtaining boundary conditions for a one-sided numerical model of thermoconvective instabilities in evaporating pinned sessile droplets of ethanol on heated substrates. In the one-sided model, appropriate boundary conditions for heat and mass transfer equations are required at the droplet surface. Such boundary conditions are obtained in the present work based on a derived semiempirical theoretical formula for the total droplets evaporation rate, and on a two-parametric nonisothermal approximation of the local evaporation flux. The main purpose of these boundary conditions is to be applied in future three-dimensional (3D) one-sided numerical models in order to save a lot of computational time and resources by solving equations only in the droplet domain. Two parameters, needed for the nonisothermal approximation of the local evaporation flux, are obtained by fitting computational results of a 2D two-sided numerical model. Such model is validated here against parabolic flight experiments and the theoretical value of the total evaporation rate. This study combines theoretical, experimental, and computational approaches in convective evaporation of sessile droplets. The influence of the gravity level on evaporation rate and contributions of different mechanisms of vapor transport (diffusion, Stefan flow, natural convection) are shown. The qualitative difference (in terms of developing thermoconvective instabilities) between steady-state and unsteady numerical approaches is demonstrated.

Chapter 25 – Gravity

The formation of droplets happens constantly on Earth, whether it is a natural phenomenon or the result of human activity. However, creating a droplet under reduced gravity conditions is different. How can the droplet be deposited onto the substrate when the fluid floats in the test cell and when the scientist floats around his experiment? This chapter discusses how to bypass the disadvantages of weightlessness and use the advantages of microgravity; it also presents a few studies performed under hypergravity using a centrifuge.

Droplets of Colloids

So far, this book has only focused on pure fluids; however, despite working with fluids with a purity of 99%, after several evaporations in the same system or on the same surfaces, a thin deposition can be observed. This deposit is due to impurities present in the fluid evaporated and by contamination during use by dust, residues, and so on, despite efforts to ensure that the fluids are as pure as possible—at best at 99.9%. After a droplet evaporation containing a small percentage of particles, why does the deposit form a perfect ring with almost all of the matter in the ring rather than in the form of a homogeneous deposit over all of the droplet wet area? This question has been answered in the study of a coffee spill onto a table by Robert Deegan. After such coffee droplets dry out, a perfect ring of ground coffee is formed on the table with an empty center. This phenomenon, known as the coffee ring effect, was explained almost 20 years ago and received renewed interest in the scientific community to explain the deposition and self-assembly of particles, as well as the pattern formation of cracks. This recent interest has led to the study of fluids containing particles from microparticles to nanoparticles to understand in detail the deposition phenomenon, not only the self-assembly of particles but also the formation of cracks that occurs in the last stage of the drying.

Liquid and Vapor Properties

In chapter 8 and 9, we saw that natural convection influences the evaporation flux rate and that there is a need for modeling correctly the global evaporation flux rate. The model proposed in chapter 9 takes into account diffusion and atmospheric convection of the vapor in the surrounding gas. This model has been validated in the literature for classical liquids such as water or ethanol for which the vapor densities are above and below the air densities, respectively. Thus, this difference in vapor density is included in the model. In this chapter, we develop an extended empirical model to satisfy other fluids and generalize the diffuso-convective model. To do so, several pure alcohols and alkanes were used.

Radiative Heat Transfer in Droplets

Evaporating droplets are usually volatile fluids that are also semitransparent to infrared rays. Radiative heat transfer is disregarded while it contributes the heat transfer from all interfaces (both substrate and liquid interfaces). Infrared cameras make it possible to observe the radiative heat flux coming from the heated substrate and passing through the semitransparent fluid and then through the surrounding gas. Thus, the temperature provided by the camera is not the real fluid temperature but an averaged temperature of a column going from the substrate to the camera. In order to access a more precise column of fluid temperature, several assumptions are needed and developed in this chapter.