Benjamin Sobac

Pattern formation in drying drops of blood

The drying of a drop of human blood exhibits coupled physical mechanisms, such as Marangoni flow, evaporation and wettability. The final stage of a whole blood drop evaporation reveals regular patterns with a good reproducibility for a healthy person. Other experiments on anaemic and hyperlipidaemic people were performed, and different patterns were revealed. The flow motion inside the blood drop is observed and analysed with the use of a digital camera: the influence of the red blood cells motion is revealed at the drop periphery as well as its consequences on the final stage of drying. The mechanisms which lead to the final pattern of the dried blood drops are presented and explained on the basis of fluid mechanics in conjunction with the principles of haematology. The blood drop evaporation process is evidenced to be driven only by Marangoni flow. The same axisymmetric pattern formation is observed, and can be forecast for different blood drop diameters. The evaporation mass flux can be predicted with a good agreement, assuming only the knowledge of the colloids mass concentration.

Triple-line behavior and wettability controlled by nanocoated substrates: influence on sessile drop evaporation.

In this article, we investigate the influence of the surface properties of substrates on the evaporation process. Using various nanocoatings, it is possible to modify the surface properties of substrates, such as the roughness and the surface energy, while maintaining constant thermal properties. Experiments are conducted under atmospheric conditions with five fluids (methanol, ethanol, propanol, toluene and water) and four coatings (PFC, PTFE, SiOC, and SiO(x)). The various combinations of these fluids and coatings allow for a wide range of drop evaporation properties to be studied: the dynamics of the triple line, the volatility of fluids, and a large range of wettabilities (from 17 to 135°). The experimental data are in very good quantitative agreement with existing models of quasi-steady, diffusion-driven evaporation. The experimental results show that the dynamics of the evaporative rate are proportional to the dynamics of the wetting radius. Thus, the models succeed in describing the evaporative dynamics throughout the evaporation process regardless of the behavior of the triple line. Moreover, the use of various liquids reveals the validity of the models regardless of their volatility. The results also confirm the recent finding of a universal relation for the time evolution of the drop mass, independent of the drop size and initial contact angle. Finally, this study highlights the separate and coupled roles of the triple line and the wettability on the sessile drop evaporation process. Data reveal that the more wet and pinned a drop, the shorter the evaporation time.

Thermocapillary instabilities in an evaporating drop deposited onto a heated substrate

The present study is an experimental investigation regarding the evaporation of ethanol drops deposited onto a heated substrate in a partial wetting situation. The originality of this work is based on the simultaneous observation of the kinetics of evaporation, heat and mass transfers, the triple-line dynamic, and thermal motions inside the drop. The triple line recedes during the drop evaporation and a spontaneous development of thermal-convective instabilities driven by the evaporation are observed. These instabilities are interpreted as hydrothermal waves induced by surface tension gradient along the free surface. An infrared technique is used to investigate the temporal and spatial dynamics of the hydrothermal waves. Results reveal a non-linear evolution of the number of waves as well as several instability regimes. A complete description of the drop evaporation with the evidence of several phases is provided. The influence of geometrical and thermal parameters has been analyzed and raised scaling law...

Triple line motion and drop evaporation: Chapter 3

Droplet wettability is altered by the substrate and thus the contact dynamics as well. The different modes of spreading are due to the interaction between the binary liquid and the substrate. These modifications are influenced by the evaporation occurring at the triple line and thus the total of evaporation. Changing the substrate modifies the surface energy and its roughness. These properties are linked to the material. Spreading and evaporation combine, so it is difficult to isolate the effect of each. In addition, changes in the substrate material can alter its thermal properties. We demonstrated in the previous chapters the link between the material properties and the evaporation rate. The triple line dynamics are thus very complex and are linked to droplet evaporation. In this chapter, we discuss theoretical concepts and recent advances involving this topic.

Triple Line Motion and Evaporation

Droplet wettability is altered by the substrate and thus the contact dynamics as well. The different modes of spreading are due to the interaction between the binary liquid and the substrate. These modifications are influenced by the evaporation occurring at the triple line and thus the total of evaporation. Changing the substrate modifies the surface energy and its roughness. These properties are linked to the material. Spreading and evaporation combine, so it is difficult to isolate the effect of each. In addition, changes in the substrate material can alter its thermal properties. We demonstrated in the previous chapters the link between the material properties and the evaporation rate. The triple line dynamics are thus very complex and are linked to droplet evaporation. In this chapter, we discuss theoretical concepts and recent advances involving this topic.

Chapter 8 – Pure Diffusion

The evaporation phenomenon is a phase change transition from a liquid to its own vapor. The molecular scale description is a transfer of molecules from the liquid to the surrounding gas, while the macroscopic scale description is a mass flux crossing the liquid/vapor interface. Evaporation occurs as soon as the vapor partial pressure in the gas is lower than the saturation pressure. This basic mechanism is called diffusive evaporation, and it always exists until the saturated conditions are not reached. Later, other supplementary heat and mass transfer mechanisms can appear, such as convections (thermo-gravitational, thermo-capillary, and thermo-solutal) or radiation. This chapter discusses these additional mechanisms and examines the theory of purely diffusive evaporation from analytical solution to approximated models. Indeed, droplet evaporation is usually considered a process that is controlled by the vapor diffusion into the environment. This is considered the basic modeling of droplet evaporation. This description appears to be very satisfactory for water evaporating into air at room temperature.

Chapter 3 – Triple Line Motion and Evaporation

Droplet wettability is altered by the substrate and thus the contact dynamics as well. The different modes of spreading are due to the interaction between the binary liquid and the substrate. These modifications are influenced by the evaporation occurring at the triple line and thus the total of evaporation. Changing the substrate modifies the surface energy and its roughness. These properties are linked to the material. Spreading and evaporation combine, so it is difficult to isolate the effect of each. In addition, changes in the substrate material can alter its thermal properties. We demonstrated in the previous chapters the link between the material properties and the evaporation rate. The triple line dynamics are thus very complex and are linked to droplet evaporation. In this chapter, we discuss theoretical concepts and recent advances involving this topic.

HEAT TRANSFER AND FLOW INSTABILITIES IN ETHANOL SESSILE DROPS UNDER EVAPORATION

Thanks to a recent increase in space resolution and temperature accuracy of infrared camera device, it’s now possible to perform thermal visualizations of sessile drops under evaporation. Using infrared techniques, we can access local thermal motions inside millimetric drops without perturbing the internal mechanisms. In the full paper, we will provide a literature review of experimental, numerical simulation and theoretical work recently perform on sessile drop evaporation. We will also detail the experimental setup which has been elaborated to realize these thermal observations. Using infrared and visible video recording, we can follow respectively the evolution of the motion inside the drop and the drop shape during evaporation. Using a heat fluxmeter placed below the drop, we can analyze the heat transfer between the substrate and the drop. We will completely describe the evaporation process based on a reference experiment and evidence the existence of several phases during this process. Then, we will dwell on the heat flux transferred to the drop during each step of the evaporation process to obtain very important information about the coupling between flow motion and heat transfer coefficient. Finally, we will present the influence of substrate temperature and drop size on the evaporation process which leads us to build a scaling law and better understand drop evaporation process.Copyright