Quantitative analysis of contact line behaviors of evaporating binary mixture droplets using surface plasmon resonance imaging

https://doi.org/10.1016/j.ijheatmasstransfer.2020.120690Get rights and content

HIGHLIGHTS

  • 1

    Contact line behavior of evaporating binary mixture droplet are examined quantitatively.

  • 2

    The surface plasmon resonance imaging technology is used for in-situ measurement of concentration of binary mixture droplets.

  • 3

    The weighted ethanol flux is quantitatively correlated with the receding velocity of the contact line.

Abstract

A sessile binary mixture droplet (BMD) has a relatively simple geometry; it is in the shape of a spherical cap. However, the contact line motion of a BMD during evaporation shows complex transient behavior that cannot be modeled using the current droplet evaporation model. The present study aims to quantitatively investigate the influence of initial ethanol concentrations on the contact line dynamics of an ethanol-water BMD. We use surface plasmon resonance imaging (SPRi) to simultaneously measure contact line motion and ethanol concentration during BMD evaporation. The visualization results show that non-monotonic contact line motions during evaporation can be subdivided into three stages: I) an initial spreading stage, II) a rapid sliding stage, and III) a moderate sliding stage. Results show behavior that is quite different from the typical motion of an evaporating DI water droplet, i.e., a simple pinning-depinning contact line motion. A weighted ethanol flux is newly introduced to consider the effect of spatial and temporal evaporative flux on the complex sliding motion of a BMD. Moreover, the weighted ethanol flux is quantitatively correlated with the receding velocity of the contact line in stage II.

Introduction

Droplet evaporation is one of the most common phase change phenomena that occurs in the technologies used in daily life. Many researchers have studied the applications of droplet evaporation in industries such as medical diagnosis, food quality assessment, medicine manufacturing, and nanochromatography [1], [2], [3], [4]. Shimoni et al. [5] used droplet evaporation using an ink-jet printing technique to make transparent conductive films. Rijckaert et al. [6] also utilized droplet evaporation with an ink-jet printing technique to fabricate superconducting nanocomposite films. Despite the many applications of droplet evaporation in different industries, the mechanism of droplet evaporation has been under debate for several decades [7,8].

In particular, the contact line dynamics of an evaporating droplet have been the subject of many studies due to its crucial role in mass transfer at the interface and the internal flow motion inside a droplet [9,10]. The majority of these studies have examined the contact line dynamics of an evaporating single component sessile droplet. Picknett and Bexon [11] distinguished the contact line behavior of an evaporating sessile droplet into two stages, namely, the constant contact radius (CCR), which is the stage I, and the constant contact angle (CCA), which is stage II; these stages are shown in Fig. 1(a). Later, other researchers reported the various contact line behaviors of a single-component droplet, such as sliding motion where both the contact angle and the radius decrease constantly [12], and stick-jump motion where the contact line is pinned repeatedly [13]. Nguyen et al. [14] suggested a theoretical expression for the lifetime of a water droplet, considering the transition of contact line motion from stage I to stage II. They used the Runge-Kutta method to predict the droplet lifetime based on the assumption of quasi-steady-state evaporation. Interestingly, they used the measured minimum receding angle as a criterion between stage I and stage II. Stauber et al. [15] suggested a scaled pinning force, fp, as an important parameter to describe the transition from stage I to stage II. The scaled pinning force can be derived from Young's equation, which is divided by the surface tension of the fluid. This parameter could be useful to describe the transition from the CCR stage to the CCA stage; however, it would be valid only for a single-component liquid droplet. Dietrich et al. [13] observed that after the CCR stage, the droplet was pinned again repeatedly by other pinning sites with each pinning strength. They called this motion the stick-jump stage and investigated the effect of surface roughness on the stick-jump stage.

Meanwhile, in the case of a BMD, the more volatile component evaporates preferentially because it has a higher saturation vapor pressure, called “selective evaporation,” which causes the concentration gradient in the droplet; therefore, it gives rise to the non-monotonic dynamics of contact line dynamics, evaporation rate, and internal flow [7]. In the past decade, researchers have paid significant attention to binary mixture droplets (BMDs) because the non-monotonic behavior of BMDs poses attractive questions in heat and mass transfer, and because their properties, such as uniform particle deposition and multi-ring patterns, have relevance to various industries [16,17]. The characteristics of internal flows in BMDs have been extensively investigated by various researchers [18], [19], [20], [21], [22]. The corresponding selective evaporation of BMDs has also been studied analytically and numerically [23], [24], [25], [26], [27], [28], [29].

In particular, a BMD exhibits non-monotonic contact line behavior, as compared to a single component droplet characterized by its CCR or CCA stage, as shown in Fig. 1(a). Sefiane et al. [30] investigated the contact line motion of an evaporating ethanol-water BMD on a polytetrafluoroethylene (PTFE) surface under the ambient conditions. They found that the contact line motion of a BMD can be categorized into three stages, as shown in Fig. 1(b), including I) the initial spreading stage, II) the rapid sliding stage where the contact angle increases to its maximum value, but the contact radius drastically decreases, and III) the moderate sliding stage. According to these results, the CCR stage does not occur while a BMD is evaporating. Therefore, the contact line of a BMD moves continuously, i.e., there is no pinning state for the contact line of BMD evaporation, unlike in single component droplet evaporation. Katre et al. [31] studied the evaporation dynamics of an ethanol-water BMD with the ethanol concentration of 20 w.t.% and the volume of 3.5±0.3 μl on the inclined heated surface. They found that while a pure water droplet remains in a pinned state at any inclination angle, the binary droplet slides at all angles of inclination. In particular, the contact line of a BMD in the receding side slides during evaporation, while the advancing side remains in a pinned state. The results of their work on the contact line motion of a BMD were qualitative. However, Sefiane et al. [32] explained how the contact line velocity was correlated with the balance between evaporation loss and hydrodynamic spreading. Nevertheless, there is still a lack of quantitative results supporting the physical relationship among contact line velocity, spreading motion, and local evaporation for a BMD.

The present study aims to investigate the contact line dynamics of BMDs quantitatively. We discuss the physical mechanisms for the contact line behavior of ethanol-water BMDs. A parameter of weighted ethanol flux is newly introduced to describe the transition of the contact line behavior correlated with the receding velocity of the contact line for BMDs. In particular, this study used SPRi due to its high-sensitivity in detecting the refractive index of the test medium to simultaneously record real-time changes in contact line motion and concentration of BMDs during the evaporation.

Section snippets

Experimental Setup

Surface plasmon resonance (SPR) is a physical process through which the surface plasmons are excited by evanescent waves. In other words, SPR is an oscillation of free electrons at the interface between metal and dielectric material, stimulated by the incident light under a total internal reflection (TIR) condition. SPR is one of the fundamental principles of refractometric sensing devices [33]. Fig. 2 shows the experimental setup used to measure the time-varying ethanol concentration and

Results and discussion

Fig. 4(a) and (b) represent the side view and bottom view images of BMD with E30%, respectively. The results are exhibited against the normalized time, t*, defined as t/ttotal, where t is the time, and ttotal is the lifetime of the droplet. The ttotal of BMD with E30% is 627 seconds. The droplet lifetime, ttotal, is defined as the duration time between the instants when the SPRi captures the signal of the liquid droplet (at t* = 0) and no signal of the liquid droplet (t* = 1) as shown in Fig. 4

Conclusions

The present study quantitatively investigated the contact line dynamics of ethanol-water binary mixture droplets (BMDs). The results show the contact line dynamics, different from the typical pinning-depinning behavior of a DI water droplet. Five cases were examined to study the effect of initial ethanol concentration on the contact line dynamics of BMD. We successfully measured the ethanol concentration within the evaporating BMD by using SPRi. Conclusions were drawn, as follows:

  • 1.

    The contact

CRediT authorship contribution statement

Chan Ho Jeong: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Writing - review & editing, Visualization, Investigation. Hyung Ju Lee: Visualization, Investigation, Software. Dae Yun Kim: Visualization, Investigation, Software. Shahab Bayani Ahangar: Writing - review & editing. Chang Kyoung Choi: Conceptualization, Methodology, Writing - review & editing, Investigation. Seong Hyuk Lee: Conceptualization, Supervision, Writing - review & editing, Investigation,

Declaration of Competing Interest

None.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1F1A1072600 and NRF-2020R1A4A4078930).

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