Experimental study on the evaporating progress of hexane lens on immiscible liquid:Spreading and receding

The change of evaporation liquid on another immiscible liquid has important guiding significance for many applications. In this experiment, the geometric temperature distribution and evaporation rate of n-hexane droplets were observed and recorded by changing the temperature of deionized water. The results show that with the increase of temperature of deionized water-based solution, the maximum diameter of n-hexane droplet spreading after titration increases gradually, while the minimum diameter of n-hexane droplet disappearing decreases gradually. Meanwhile, the evaporation rate of nhexane droplet is constant during the whole evaporation process. It should also be mentioned that if the base solution is changed from deionized water to a certain concentration of salt solution, the maximum diameter of n-hexane droplet spreading will be reduced, and the evaporation intensity will be relatively reduced. These experimental results will give us a better understanding of the mechanism and characteristics of droplet evaporation.


Introduction
Droplet evaporation is a common phenomenon in nature. It is of great significance to study it. Many scholars have conducted in-depth and extensive research on droplet evaporation, and literature [1] and [2] have made a detailed summary of the research in this aspect. The evaporation of liquid droplets is one of the research aspects ， but the evolution of an evaporating droplets on a nonmiscible liquid substrate has only recently received attention, which is of practical importance in many industrial forces, the latter two states are often referred to as "Lens".In the early stage of lens research, we focused on the theoretical analysis and measurement methods of lens geometry, Based on the Young−Laplace equation, Neumann's rule is constructed to describe the geometry of lens。When an alkane lens is placed on the surface of a surfactant solution, its three-phase contact line has an associated line tension, which plays a crucial role in determining the relative size [13] . Pujado  for the geometric structure of lens considering this factor [14] .
Emelyanenko et al. established the three-phase contact angle calculation formula of benzene lens based on the isoline of separation pressure. It was found that the spreading and shrinking process of benzene lens was the result of several different types of surface forces, among which the moisture content of benzene lens had a greater influence [15] .Ooi et al. studied the floating mechanism of lens by CT scanning technology. It was found that the shape of lens was mainly affected by the surface tension, and the contact angle would change with the volume of lens due to the flexibility of the base liquid interface [16] . Burton  Young−Laplace equation to analyze lens geometry of liquid [17] , and this lens profile calculator, together with a measurement of the lens radius for a known volume, provides a simple and convenient method of determining the spreading coefficient (S) of a liquid lens system. Nikolov et al. also reviewed the related research of liquid lens, proposed an accurate method to measure the three-phase contact angle of micro lens by using reflected light interference, verified the effectiveness of Neumann rule for micro lens, and evaluated the three-phase contact angle and the radius of upper and lower interfaces of two kinds of (spreading and non spreading) lens [3] .
Another focus on the study of liquid lens is its dynamic dissolution, spreading and evaporation process. Kim et al.
studied the mixing mechanism of miscible liquid, and found that the lens structure and Marangoni convection can also be produced before the miscible liquid is mixed, and established a theoretical model to predict its spreading time, geometric scale and Marangoni convection velocity [18] .  [19] . Nosoko et al. studied the evaporation process of n-pentane lens on water surface, and found that the static time of water, the purity of n-pentane, the temperature difference of environment and other factors can significantly affect the evaporation time and geometric characteristics [20] . Shimizu et al. studied the evaporation process of n-pentane and trifluoroacetyl chloride droplets on water surface under moderately elevated pressures(0.48 MPa ) , and Data obtained were processed to yield information on parameters of practical importance [21] .
Gelderblom et al. studied the evaporation process of small droplets on the liquid surface, and revealed the flow law in the three-phase contact line region by solving Stokes equation [22] . Sun et al. Studied the evaporation process of toluene and n-hexane lens on the deionized water surface, and found that both of them experienced the spreading stage and the shrinking stage in turn, and the shrinking speed increased significantly when the dimensionless time was 0.7 ~ 0.8 [23] Phan et al. Studied the floatability of water droplets on the oil surface, and found that the stability of floating droplets is affected by the interfacial tension between various media, oil density and water droplet volume. The water droplets are more stable after adding surfactant, which indicates that the surfactant plays an important role in the stability of water droplets [24] , Bresme et al. used molecular dynamics method to simulate the wetting behavior of nanoscale liquid lens on the liquid interface, and found that Neumann rule in the macro scene can also accurately describe the wetting behavior of nanoscale lens [26] . Bertrand et al. measured the wetting behavior of different alkanes on water surface by ellipsometry, and found that there were two wetting transitions of alkanes. Based on the experimental data, the wetting transition function with alkane chain length and temperature as variables was constructed [27] .

Experimental apparatus
The experimental data involved in this paper are measured by three relatively independent experimental devices, including real-time image measurement system, surface temperature measurement system and evaporation rate measurement system. The real-time image acquisition system consists of a 3D video microscope (rh2000, hirox, Japan), a computer, a micro feeding pump and a culture dish.
The 3D video microscope is connected with the computer to transmit the collected image to the computer. The diameter of the culture dish is XX mm and the depth is XX mm In order to ensure the accuracy of titration volume and the experimental requirements, the syringe is equipped with a 250 mm long metal probe.
The surface temperature measurement system is composed of infrared thermal imager (x6520, FLIR, USA), computer, culture dish, micro feeding pump, thermocouple and data recorder. Among them, the resolution of the infrared thermograph is 640×512 and the temperature resolution is 0.05k, two T-type thermocouples are used, which are connected with data recorder. The thermocouples are calibrated carefully (±0.1℃) before use, and inserted into the medium of culture dish to monitor the temperature change of medium.
The evaporation rate measurement system is composed of high precision electronic balance, computer, lens, culture dish, micro liquid pump, thermocouple and data recorder.
The culture dish is placed on the tray inside the highprecision electronic balance. The high-precision balance is connected with the computer, and the quality reading is directly imported into the computer for recording.

Experimental materials
The NaCl used for preparing salt solution was purchased from (purity: > 99.8%, Shanghai Macklin Biochemical Co. Ltd. China). High precision electronic balance was used to weigh NaCl and deionized water, and salt solutions with mass fractions of 6%, 12%, 18% and 24% were prepared and put in closed containers for standby.

Experimental procedure
Before the experiment, most of the dust and dirt on the surface of syringe, culture dish and thermocouple probe were removed by ultrasonic cleaning instrument, and then cleaned with acetone, alcohol and distilled water in turn.one after another, for five minutes each, followed by air flow treatment for drying.
During the experiment, the container containing the base solution was put into a constant temperature water bath and heated or cooled to a specific temperature ( Table 1)

Data processing
The diameter change of n-hexane lens during evaporation was obtained by Image J software. The video file of nhexane lens evaporation process recorded by 3D video microscope was imported into image J, and the change of nhexane lens diameter with time was measured by using the measurement function of the software. The evaporation rate of n-hexane is equal to the mass change rate of n-hexane droplet (n-hexane lens + base solution) minus the mass change rate before n-hexane is added.

Characteristics of droplet spreading and diameter variation
When n-hexane droplets are exposed to salt solutions at different temperatures (concentration: 18%), the diameter change of n-hexane lens is shown in Fig. 2a. We can find that the change of the diameter of n-hexane lens can be divided into three stages: 1) spreading stage, which is similar to the change of droplet shape on the solid plane. In  As the total mass of stage 2 and stage 3 changes linearly, we can infer that the evaporation rate of n-hexane is relatively constant in the whole process, that is, the volume reduction

Evolution of lens trilinear in nhexane
The morphology and evolution of n-hexane lens will change with the evaporation process, and each stage shows different characteristics (Fig. 5). In the spreading stage, n-hexane first Next to the region is the region where eddy current is generated and affected. 1) the reduction rate of n-hexane diameter increases significantly (Fig. 5F), which can also be seen from Fig. 2 and Fig. 3 It was observed that: 2) the diameter of micro droplets produced at the edge was significantly reduced (Fig.   5g). When the diameter decreases to a critical value, the lens structure of n-hexane disintegrates instantaneously and a thin film is formed. After evaporation, all the small droplets and films will remain on the surface of deionized water, and finally form three distinct areas, as shown in Fig. 6.

Discussion
For a system consisting of hexane and DI water or NaCl solution, both S and A are positive, and pseudopartial wetting is expected. In the experiments by bgil, K. [28] , hexane droplets on the surface of fresh DI water remained the shape of liquid lenses during both the advancing and receding stages in an environment with unsaturated hexane vapor.

Reasons for the difference of n-hexane diameter
In most cases, the surface tension will gradually decrease with the increase of temperature. However, we fitted the surface tension of deionized water and n-hexane with the change of temperature between 0℃ and 5℃, and found that the change rates of surface tension of deionized water and n-hexane with the change of temperature were -0.1525mn/m℃ and -0.1062mn/m℃, respectively.
Therefore, the influence of the temperature of deionized water on the geometry of n-hexane is greater than that of n- The diameter of n-hexane lens is closely related to its volume and contact angle. The droplet volume is determined by two aspects: one is the evaporation process, the other is the separation of tiny droplets in the boundary region.Following the work given by Sun and Yang [29] , the resultant force, ΔF, applied to the contact line can be expressed as

Reasons for linear variation of evaporation rate
It can be seen from Figure 3 that the mass of n-hexane lens decreases linearly, indicating that the evaporation rate of nhexane is relatively constant. Generally, the evaporation process of droplets can be described by Fick's law, the hexane lens starts to evaluate due to the difference of hexane concentration between the surface of the lens and the air, which can be described by Eq. (1). (1) where, is the evaporation rate of hexane lens, g/s; Aw is the surface area of droplet, m2 ； Mw denotes the molecular weight of hexane, g/mol; , denotes the vapor concentration on the surface of droplet and in the air, respectively, mol/m3; Tw and Ta are the surface temperature of the lens and the temperature of air, respectively, K; p is the local absolute pressure, Pa; R is the universal gas constant, 8.3145 J/(mol·K).
Therefore, if the evaporation process of n-hexane lens conforms to Fick's law, the two phenomena seem to be contradictory, and the evaporation rate should gradually decrease, which is inconsistent with the experimental results.
Because the experiment is carried out in an open environment and the ambient temperature is constant.
Therefore, D∞can be considered as a constant. DS is directly related to temperature. The temperature change during evaporation is observed by infrared thermal imager.
The surface temperature of n-hexane during evaporation  Figure 7. It can be found that the surface temperature of n-hexane decreases with the evaporation process.
kc denotes the mass transfer coefficient, kg/s, which can be obtained from the Sherwood number correlation as follows, where dw is the diameter of droplet, m; Sc is the Schmidt number; Re is the Reynolds number; Dc is the diffusion coefficient of hexane vapor in the air, m 2 /s; ud is the velocity of droplet, m/s.

Influence of salt solution concentration
The

Principle of three phase line instability
The surface temperature of deionized water and n-hexane lens are different, so we can't calculate them according to the same temperature. This process is a dynamic balance rather than a static force balance process.
The reason why the instability of lens three-phase system can occur may be due to the flow. At this time, the surface tension size at the three-phase line has a problem, which may change the surface tension distribution.
The development and evolution of convective patterns induced by evaporation is a complex problem. For a volatile fluid, the vertical temperature gradient results from the evaporation itself and from the coupling between the free surface and the surroundings. If the temperature decrease is large enough, buoyancy forces and surface tension variations may overcome stabilizing forces, and convection begins [30] .

Conclusions and Prospect
We found that the change of the diameter of n-hexane lens can be divided into three stages: 1) spreading stage; 2)