Aggregation-Induced Emission Luminogen-Based Direct Visualization

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AIEgen based direct visualization of concentration gradient inside an evaporating binary sessile droplet Xin Cai, Ni Xie, Zijie Qiu, Junxian Yang, Minghao He, Kam Sing Wong, Ben Zhong Tang, and Hui-he Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09008 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Applied Materials & Interfaces

AIEgen based direct visualization of concentration gradient inside an evaporating binary sessile droplet. Xin Cai1, Ni Xie2, Zijie Qiu2, Junxian Yang1, Minghao He1, Kam Sing Wong5, Ben Zhong Tang*234 and Huihe Qiu*1 1

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science

and Technology, Clear Water Bay, Kowloon, Hong Kong 2

Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research

Center for Tissue Restoration and Reconstruction, Institute of Molecular Functional Materials, Institute for Advanced Study, State Key Laboratory of Neuroscience, Division of Biomedical Engineering and Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 3

Guangdong Provincial Key Laboratory of Brain Science, Disease and Drug Development,

HKUST-Shenzhen Research Institute, Nanshan, Shenzhen 518057, China 4

Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key

Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China

ACS Paragon Plus Environment

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Department of Physics, The Hong Kong University of Science and Technology, Clear Water

Bay, Kowloon, Hong Kong KEYWORDS: Binary droplet, evaporation, different volatilities, concentration gradient, AIE.

ABSTRACT: In this study, the concentration gradient inside evaporating binary sessile droplets of 30, 50 and 60 vol% Tetrahydrofuran (THF)/water mixtures were investigated. The 5µL THF/water droplets were evaporating on a transparent hydrophobic substrate. This is the first demonstration of local concentration mapping within an evaporating binary droplet utilizing the Aggregation-Induced Emission material. During the first two evaporation stages of the binary droplet, the local concentration can be directly visualized by the change of fluorescence emission intensity. Time-resolved average and local concentrations can be estimated by using the preestablished function of fluorescence intensity versus water volume fraction.

INRODUCTION The study of mini/micro-sized binary droplet evaporation has attracted much attention for their various applications in ink-jet printing 1, thermal management in cooling systems 2, DNA mapping 3-4 and blood tests for disease diagnosis5 . For decades, the evaporation dynamics have been investigating by numerous studies

6-12

. Compared to the pure liquid droplet

10, 13-14

, the

evaporation behavior of a binary droplet is much more complex. To have a better understanding on the process, extensive studies have been done. Studies of multicomponent droplet evaporation on heated and non-heated flat surfaces have been conducted experimentally 15-17; The influences of substrate temperature on the evaporation characteristics of droplets on hydrophobic and superhydrophobic surfaces were experimentally investigated by Dash et al. and they found out that the


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higher the hydrophobicity of the substrate, the lower the evaporation rate of the drop 8. BourgesMonnier et al. investigated the influence of evaporation on the contact angle of the binary water and n-decane18. Sefiane et al. studied the evaporation rate of the binary water-ethanol droplet, by measuring the dynamic contact angle and droplet dimensions, which are base width and volume, as function of time 19. Christy et al. investigated the flow field inside of an evaporating ethanolwater droplet during three evaporation stages by using particle image velocimetry (PIV) technique 20. Based on previous studies, the evaporation process of droplet with binary mixtures of a volatile organic liquid and water can be divided into three stages: in the first stage, the volatile component preferentially evaporates, during the second or say transitional stage, the contact angle of droplet increases upon the drop base decreasing, and the third stage is almost similar to the evaporation of pure water. It has been found out that most of the volatile component has evaporated until the second stages finished 19, 21. During the first two stages of the evaporating binary droplet, subsequent studies have shown that the dynamic behavior of the droplet can arise from the temperature or concentration gradient 22

20,

. Although intensive studies have been done to understand this fundamental process, there are

still many unsolved questions on the local concentration distribution and local surface tension differences during the evaporation process, which related to the complexities of internal flow and contact line dynamics. Countable studies were carried out to investigate the concentration evolution of evaporating binary mixtures recently. Dehaeck et al. mapped the local ethanol concentration at different moments of the evaporating cocktails by detecting the refractive index of different components inside the cocktail in macrosize

23

. By using the acoustic method, Pin

Chen et al. successfully tracked the volatile components concentration at the bottom of a droplet


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of water/ethanol and water/1-butanol mixtures during evaporation and they monitored the alcohol concentration evolution versus time 24. However, the local concentration gradient inside a mini/micro-sized multicomponent droplet during evaporation is still not well understood. With the development of Aggregation-Induced Emission (AIE) luminogen (AIEgen), the AIEgen has been applied into various applications and fields, for example, OLED biological imaging

27

and detection

28-29

25

, bio-probing

26

,

. In the AIE process, non-emissive luminogens are

induced to emit by the aggregate formation. The AIEgens are usually non-emissive when its molecules are dissolved in a good organic solvent, e.g., tetrahydrofuran (THF), acetonitrile, ethanol and acetone, but display strongly emissive properties when they aggregate in bad solvent such as water

30-31

. Generally, the fluorescence emission of AIEgens in binary mixtures can be

enhanced with the water volume fraction increased. The change of concentration-dependent fluorescence emission of AIEgens in binary mixtures provides the information about the local concentration profile inside the solution. In this study, we propose to track the fluorescence intensity profile from AIEgens for probing the local concentration gradient of the evaporating binary THF/water droplets. The pre-established function of fluorescence intensity versus water volume fraction in THF/water mixtures gives us the possibility to depict the local THF concentration inside the binary droplet during its evaporation process. MATERIALS AND METHODS Material Polymer-AIEgen named P1a/2a/3 (p-AIEgen) is directly synthesized and donated by AIEgen Biotech Co., Limited with Mw 7300; Mw/Mn 1.8. The molecular structure of p-AIEgen, the thermal stability, and the relative emission intensity (I/I0) versus the composition of the THF/water mixture of p-AIEgen have been reported in the previous literature 32. Tetrahydrofuran


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(THF) was distilled from sodium benzophenone ketyl under dry nitrogen immediately prior to use. DI water are shorten as water used in these experiments. Preparation of polymer-AIEgen nanoaggregates in binary THF/water solutions A stock solution of p-AIEgen in THF with a concentration of 20 mM was prepared. Aliquots (1mL) of the stock solution were transferred to 10 mL volumetric flasks. After adding appropriate amounts of THF, water was added dropwise under vigorous stirring to furnish 200 µM solutions with defined fractions of THF (30, 50, 60 vol %). To date, most of the reported AIE-active fluorescent polymer nanoparticles (PNPs) were spheres with diameters ranging from tens to hundreds of nanometers

33

. It has been demonstrated that the morphology of PNPs is

critical for their biomedical properties and applications. The distribution of particle diameters of p-AIEgens in this work has been investigated using dynamic light scattering (DLS) technique. The diameter of p-AIEgen in THF/water solution are less than 400nm, when water fractions ranges from 40% to 90%. The particle size of p-AIEgen is smaller than the common used seeding particles. The p-AIEgen nanoaggregates used in this study have been reported to show a high thermal stability under 400℃ 32. Experimental procedure Figure 1 shows the layout of our experimental setup. In this study, binary sessile droplets were evaporating on a transparent hydrophobic substrate. The hydrophobic substrate was fabricated utilizing following procedures: fabrication of the gold electrodes and hydrophobic treatment of the substrate surface. The transparent substrate was made of ITO glass. The 100nm ITO layer can be act as a resistive heater and two paralleled gold electrodes were deposited on the top of ITO surface by physical vapor deposition (PVD) method. To obtain hydrophobic surface, the sample

was

immersed

in

a

hexane

solution

of

0.5

wt

%

1H,1H,2H,2H-


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perfluorodecyltrichlorosilane for 1h, followed by heat treatment at 180 °C in an oven for 1h. After surface treatment, pure water is found to have a static contact angle of 115 ± 4°on the substrate as shown in Figure 2. The experiments were conducted at a room temperature of 21 °C and relative humidity of approximately 40%. A DC power supply was used to heat the substrate to obtain different surface temperatures by varying the input power.

Figure 1. Schematic of experimental set up employed to visualize the fluorescence intensity inside the evaporating droplet.

Figure 2. Static contact angel of 5µL DI water droplet on the substrate.


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The base binary solutions were mixtures of deionized water and THF and three THF concentrations at 30 vol %, 50 vol % and 60 vol % were used. The initial volume of the droplet is controlled at 5µl generated by a micro syringe (Hamilton, 10 µl 700 series hand fitted MICROLITER syringe). For all of the experimental cases, droplets were placed on the geometric center of the substrate surface. To investigate the concentration gradient inside the binary droplet during evaporation, the base binary solutions were seeded with fluorescent p-AIEgen nanoaggregates. A tunable Ti:Sapphire fs Laser was used as an excitation source for the p-AIEgen. The excitation wavelength of the laser was adjusted to 390nm. A 420nm filter was inserted in front of camera lens to eliminate the background excitation. By using a set of concave and convex cylindrical lens to expand the laser beam, a laser light sheet with a thickness less than 0.03mm was created and illuminated the center vertical plane of the droplet. The direction of the light sheet was from the bottom of the transparent substrate therefore the laser reflection on the droplet surface can be reduced. The laser provided sufficient illumination at around 14mW for this study and liquid heating due to laser illumination can be ignored at this power level. A side view CMOS camera (Canon 60D) mounted with a zoom lens (Zoom 6000, Navitar Inc.) via a CF-C adaptor observed the illuminated section of the droplet. The dynamic shooting video was captured at a rate of 60 frames/s. Three times repeated tests were conducted for each set of experimental conditions in order to ensure the repeatability of the experiments. RESULTS AND DISCUSSION The fluorescence characteristics of polymer-AIEgen nanoaggregates In this study, fluorescence intensity change of p-AIEgen nanoaggregates is used to detect the concentration gradient within the evaporating sessile THF/water droplets. The p-AIEgen is favor


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to organic solvent and it is soluble in THF but not in water. Therefore, we examined the fluorescence behavior of p-AIEgen in THF/water mixtures with different water fractions to induce the aggregation form, as shown in Figure 3a. The p-AIEgen emits faintly in the THF solution but the green emission intensity is gradually enhanced with the water fraction increased. The mechanism is explained by restriction of intramolecular motion (RIM) theory. Simply put, the rotor of p-AIEgen is restricted due to the poor solvent, here is water, induced to the solvent mixture. Upon exhibiting the rotation and vibration of the phenyl rings and the polymer chain of p-AIEgen, the non-radiation relaxation of the excited state is block but allowing the tunnel of radiation relaxation. Hence, the emission is observed and detected in higher water fraction or lower THF fraction. By converting the images to 8-bit grayscale and read the intensity feature using the Matlab, the average fluorescence intensity versus water volume fraction curve can be calibrated in Figure 3b. These results are given from three times measurements performed under the same condition. As shown in Figures 3(a) and 3(b), p-AIEgen emits weak light when the water fraction ( ) is less than 50 vol%, its emission intensity increases significantly when is above 60 vol%. The highest emission intensity is achieved at 90 vol% of water. Then the fluorescence intensity decreases when the reaches to 98%. The explanation indicates that, at water fraction is very close to 100% and THF is almost 0%, the aggregates become so large that the effect of selfabsorption causes the decrease of the effective emissive species concentration in the mixture 32. A linear fitting of normalized intensity (I/I0) versus water fraction is given in the region from 40% to 90%. The gradually increasing of emission intensity in this region offers us possibility to investigate the concentration evolution of THF/water mixture evaporation.


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Figure 3. (a) Screenshots of fluorescence photographs of P1a/2a/3 in THF/water mixtures with different water fractions ( ). Photographs are taken under a UV lamp with excitation wavelength at 365 nm. (b) Plot of fluorescence intensity (I/I0) versus the of the THF/water mixture of P1a/2a/3.

Initial contact angle of sessile THF/water droplets The binary droplet was gently placed on the hydrophobic surface using a micro syringe. By measuring the contact angle in the first frame of the video after the drop has been deposited on to the substrate, the initial contact angle was obtained. For different drop concentrations, the initial contact angle decreases as the concentration of THF is increased because the volatile THF has a surface tension of 26.4 mN/m which is much smaller than the surface tension of water (72.6 mN/m) at ~21℃. According to Young’s equation,

,

(1)


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at the constant surface free energy of solid , the smaller the surface tension of the liquid the smaller contact angle will be. The initial contact angle of the THF/water drops decreased from 55°to 50° as the concentration of THF increased from 30 to 60% (vol%) at room temperature ~21℃. Evaporation process of sessile THF/water droplets Based on previous studies, the entire evaporation process of the binary droplet shows three different stages. The first and second stages are affected by volatile substance inside the binary droplet and the third stage dominated by pure water evaporation

34

. The simplified schematic

illustration of the contact angle evolution of an evaporating binary drops has been shown in Figure 4

19

. In the first stage, the contact angle decreases because of the volatile component

preferentially evaporates; during the second or say transitional stage, the contact angle of droplet increases upon the drop base decreasing; in the third stage, the contact angle decreases slowly towards the end of the evaporation. The purpose of this study is to investigate the concentration gradient, so the major stages discussed in this present study, are the first and second stages.

Figure 4. Schematic illustration of the contact angle evolution of the evaporating binary sessile droplets.


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To study the concentration gradient within the evaporating sessile THF/water droplets, the 5 p-AIEgen seeded droplets of three THF concentrations at 30 vol %, 50 vol % and 60 vol % were left evaporating on a clean hydrophobic substrate. The field of view of the optical system are calculated to make sure the boundary of the droplet is not missing in the photograph. Droplets are with initial volume of 5L and with diameter about 2mm when suspended. The sensor size of the camera (Canon 60D) is 22.2mm in width and 14.8mm in height. When the focal length is 35mm, the half angle of view can be calculated which is 11.9°. As the working distance of the Navitar Zoom 6000 is 36mm. The field of view can be calculated as 7.6mm in height and 10.1mm in width. Figure 5 presents the snapshots of the evaporating THF/water droplets seeded with p-AIEgen. The reflection of the droplet on the substrate are marked in Figure 5. From Figure 5(a) and 5(b), it can be seen that the field of view of the optical lens used in this work can cover the whole droplet area.

Figure 5. Snapshots of THF/Water droplets evaporation (a) in dark room and (b) under nature light. The reflection of the droplets have been marked. Droplets are both with initial concentration of 30%THF and initial volume of 5µL. From the two images, it can be seen that


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the field of view of the optical lens used in this work can cover the whole droplet area.

Figure 6 (a)-(f) shows the snapshots of the evaporation process of THF/water droplets containing different THF concentration at room temperature (~21℃) and on a heated surface (~35℃). The concentration mapping of the process are also presented in Figure 6. The different intensity levels indicate the different concentration inside the droplet. Emission intensity can be enhanced upon the increase of relative water volume fraction or decrease of THF volume fraction. At 30 vol% of THF, the droplet gives off a more intensive fluorescence compared to the droplets of 50 vol% and 60 vol% of THF at the initial step T=0s. This fluorescent property of droplets is agreed with the previous results shown in Figure 3.


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Figure 6. Snapshots and concentration mapping of binary THF/Water droplet evaporating process at different THF concentrations and thermal conditions. Droplet contains (a) 30 vol% THF + 70% water, (b)50 vol% THF + 50% Water, (c) 60 vol% THF + 40% Water at room


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temperature (~21oC) at T=0s, (d) 30 vol% THF + 70% Water, (e) 50 vol% THF + 50% Water, (f) 60 vol% THF + 40% Water on a heated surface (~35oC) T=0s. The reflection of the droplet on the substrate can be seen in the snapshots.

For these six cases showing in Figure 6, in the first and second stage of evaporation, the emission intensity of the drop is increasing because of the preferential evaporation of the volatile THF. It is known that the vapor pressure of THF ( ~19.0 kPa at 20℃) is higher than that of water ( ~2.8 kPa at 20℃) so THF will be referred to as volatile and evaporate faster than water at the droplet interface leading to the mixture surface tension increase. The drop surface, the free outer shell liquid layer within the liquid–vapor interface, has higher intensity compared to interior area, which indicates that the volatile THF can be transported to the surface of the droplet and evaporates first. This leads to the decrease of THF concentration on the drop surface endowing the possibility in detecting of the fluorescent intensity. The concentration gradient mapping are giving in Figure 6 (a) to (f). The color chart on the right represents the water concentration of the droplet. To investigate the fluorescence intensity profile, all images were converted to 8-bit grayscale and read the intensity levels using the Matlab. It is possible to determine the water (THF) concentration from the fluorescence intensity profile. The results are deduced from the relationship between fluorescence intensity and volume concentration of water, as shown in Figure 3b. To study the average concentration of the whole drop, the average fluorescence intensity inside the droplets were calculated. Figure 7 shows the measurement of normalized average intensity (I/I0) of the drying droplets at different time steps. The average intensity are calculated from the intensity of every pixel inside the droplet and was normalized with respect to the intensity profile


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of pure THF containing p-AIEgen nanoaggregates. Dynamic contact angle evolution with time are also presented in Figure 7. In the first two stages of binary droplet evaporation, the contact angle decreases in the first stage, then starts to increase to a certain value in accompany with the triple line depinning in the second stage

19, 34

. Combing the normalized average intensity in

Figure 7 and the relationship between fluorescence intensity and concentration of water shown in Figure 3b, the average concentration evolution with time during evaporation of droplets can be determined in Figure 8. All the results are the average of three times of measurements.

Figure 7. Normalized average intensity (I/I0) and contact angle of the drying droplets at different


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time steps for the droplets with initial concentrations of (a) 30 vol%, (b) 50 vol%, and (c) 60 vol%.

Figure 8. Average concentration evolution with time during evaporation of droplets of (a) 30 vol%, (b) 50 vol%, and (c) 60 vol% of THF under different thermal conditions.

From Figure 7 and 8, it is observed that the decrease of the THF concentration of the droplet is indicated by the increasing normalized average intensity during the THF/water droplet evaporation. When evaporating the droplet with initial concentration of 30 vol% THF, as shown in Figure 7(a) the initial fluorescence intensity of the whole drop is already high enough compared to the droplets with initial concentration of 50 and 60 vol% THF in Figure 7(b) and 7(c). The normalized average intensity (I/I0) increases gradually with the increasing time steps due to the THF concentration decreasing. Then I/I0 slows down when it comes to a certain value


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which means the decreasing of THF slows down. This is because the majority of THF have already been evaporated, the concentration of THF at the droplet surface layer and the concentration of THF in the drop bulk have decrease to a certain value. The residual THF are transported to the drop surface to evaporate with water mainly by diffusion effect but not dominated by concentration gradient anymore. The normalized average intensity curves are more gently during 0~10s in Figure 7(b) and 0~20s in Figure 7(c). The reason is that the fluorescence intensity of the THF/water mixtures is relatively low at the water fraction below 60% (Figure 3b). When evaporating the droplet with initial concentration of 50 and 60 vol% of THF, initial water concentration of 50 and 40 vol%, the normalized average intensity increases in a slow fashion until the water concentration reach to 60% follows a significant increase at water fraction larger than 60%. So far the second (transient) stage of binary droplet evaporation is of most interest

20

, previous

studies assumed that most of the volatile component evaporate during the first two stages and the local volatile component concentration should be different but cannot be measured during this stage 34. The triple line depinning moments were marked in Figure 6(a)-(f). At the beginning of evaporation (T=0s), the emission intensity of the droplet surface is higher than the one near the contact line (edge area). Before the triple line depinning, there can be seen a sudden intensity increasing near the edge area (Figure 9). This phenomenon can be explained as following: after placing the drop onto the substrate, the THF molecules tend to move toward the liquid/air interface because of the high vapor pressure and lower surface tension of THF compared to these of water. Thus, the surface of the drop was enriched with THF molecules, which resulted in a decrease of surface tension of the mixture drop. During the evaporation, the liquid-air interface becomes water rich due to the preferential evaporation of THF and higher water fraction leads to


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higher fluorescence intensity. We assume the concentration of THF along the drop surface is , and ∆,

!"# $

is the decrease of THF caused by evaporation. Deegan et al.35 found

that when the contact line is pinned, a convective radial flow of liquid from the center to the edge occurs to replenish the liquid that is removed from the edge during the evaporation. Then THF rich liquid is constantly replenished from the center of the drop to the edge area, ∆,!%$#&' represents for the replenishment of THF to the drop edge. As a result, during the 1st evaporation stage, the concentration of THF at the edge is ( ∆,

!"# $

) ∆,!%$#&' ,

which is higher than the THF concentration at the apex of the liquid surface ( ∆,

!"# $

. Thus the higher THF concentration at droplet edge leads to lower

fluorescence intensity there.

Figure 9. Snapshots of binary THF/Water droplet evaporating at the moment when triple line begin to shrink. The part in the red boxes are zoom in by side. Water concentration mapping of the zoom in part are also given.

Furthermore, higher fluorescence intensity represents the lower THF concentration, which indicates the higher surface tension. So from the intensity profile we can see that: at the


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beginning of evaporation, there is a surface tension gradient along the drop surface layer. The apex area has a higher surface tension *+ than the edge area *,- , and there exists a surface tension difference *+ ( *,- > 0. Driven by this existing surface tension difference, more volatile THF can be transported to the surface layer for evaporation. As the evaporation going on, the contact angle decreased, due to the volatile THF works its way to the surface for preferential evaporating. Meanwhile the intensity of the apex and edge area are increasing which indicates the THF evaporation leads to the local surface tension increasing. From the previous studies what we already know is that when the flatten droplet cannot sustain a high surface tension, the drop triple line begin to shrink. Here in our study, a sudden increase of intensity near the edge can be observed before the triple line shrink, shown in Figure 9. This phenomenon was first observed in our experiment and can gives us more information about the transport process within the droplet under this evaporation stage. The increase of fluorescence intensity indicates the decrease of THF concentration. As we discussed before, during the 1st evaporation stage, average concentration of THF decreases and the THF rich liquid is constantly replenished from the center of the drop to the edge in order to maintain the pinned triple line. Once the transferred THF from droplet center to the edge are not enough to replenish the THF removed by evaporation at the edge, the concentration of THF at the edge decrease and the fluorescence intensity increase under this situation, and followed by a retract of the triple line. Since the solutal Marangoni effect mainly dominating the vortices during the first evaporation stage of binary droplet 7, and the solutal Marangoni number Ma& ∆*2 3/5 6 ,

(2)


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is used to study the influence of solutal Marangoni effect on internal flow patterns. 3, 5 and 6 in Eq (2) are the drop base radius, viscosity of mixture and diffusion coefficient in binary mixture. ∆*2 is the surface tension difference and calculated by ∆*2 *2," *2,,- and *2,"

( *2,,- , where

represent the surface tension at the top of droplet and the edge of droplet. Until

now, the measurement of local surface tension along the droplet surface is difficult for us. When calculated the surface tension difference, most previous studies used the assumed values under safety condition. Our study here, offers the possibility to obtain the local surface tension difference by calculating the concentration gradient from the intensity profile. Effect of the heating on the evaporation of sessile THF/water droplets According to Figures 7a-7c, the black line represents the droplet evaporation at room temperature of 21℃ and the red line represents the cases on a heated surface ~35℃. As the substrate was heated to 35℃ the curves representing normalized average intensity (I/I0) versus time deviate from that of the room temperature cases. In our experiment a fluorescence intensity enhancement is observed when the substrate was heated. Hence the higher substrate temperature enhanced the THF evaporation of the binary droplet during the 1st and 2nd evaporation stages. The local THF concentration gradients of the evaporating droplet were also studied in this work. As shown in Figure 6, when the droplets of 60% THF are evaporating at room temperature ~21℃ (Figure 6(c)), in the first regime of evaporation, the emission intensity is stronger at apex but gradually diminished along the droplet surface toward the edge. However, when the droplets of 60% THF were left on the heated surface at 35℃ to evaporate (Figure 6(f)), an enhanced intensity on the liquid-air interface can be observed, especially near the contact line area compared to the experiment at room temperature. To study the intensity profile of the droplet


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surface layer, the surface of the droplet were divided into three areas of equal width as shown in Figure 10. The number 1 and 3 areas are on the drop surface near contact line, number 2 area is at the droplet apex. Fluorescence intensity of different surface area are plotted at different time steps of evaporation in Figure 11. The 1,2 and 3 in the x-axis are the surface area number correlated to the schematic in Figure 10. Each dot is obtained from the average intensity of the specific area. From Figures 11(a)-(c), the normalized intensity of different areas of the evaporating droplet with initial concentration at 60 vol% THF is observed. At T=0s, shown in Figure 11(a), the normalized intensity of 1,2 and 3 are higher in the heating case ,labelled in red dots, than them, with black dots, in the room temperature case. Compared the heating cases with the non-heating cases, the increase of normalized intensity ∆7 of areas 1,2 and 3 are also plotted in the right Y-axis in Figure 11. It is worth mentioning that the increase of normalized intensity of areas 1 and 3 are larger than the increment in apex area 2. During the evaporation, shown in Figures 11(b) and 11(c), the value of ∆78 and ∆79 are larger than ∆7: . Similar phenomena can be found in Figures 11(d) to 11(i). According to this, it seems that the higher substrate temperate enhance the THF evaporation on the droplet surface layer, especially near the contact line area during the first two stages.

Figure 10. Schematic of three area created on the surface layer of the droplet. The area 1 and 3 are on the surface near the contact line and area 2 is near the apex of the droplet.


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Figure 11. Plots of fluorescence intensity of different areas on the droplet surface layer at different times of evaporation. The droplets contain (a)-(c) 60 vol%; (d)-(f) 50 vol%; and (g)-(i) 30 vol% THF were studied. The surface area number shown in X-axis here are correlated to the surface area defined in Figure 10.

CONCLUSIONS This work is the first demonstration of local concentration mapping inside an evaporating binary droplets utilizing the Aggregation-Induced Emission material. In this study, concentration gradients of the evaporating binary THF/water droplets were visualized and investigated by tracking the fluorescence intensity profile of p-AIEgens. Using the function of fluorescence


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intensity versus water volume fraction in THF/water mixtures, the average concentration kinetics of the evaporating droplets can be determined. Fluorescence intensity difference along the droplet surface layer can be observed during the first and second evaporation stages, which can be translated as the local concentration. This offers the possibility to obtain the local surface tension difference on the drop surface. We also studied the evaporation of sessile THF/water droplet on a heated surface. Fluorescence intensity enhancement had been observed when the droplet was evaporating on the heated substrate. Hence the high substrate temperature enhances the THF evaporation of the binary droplet during the first two evaporation stages. Moreover, the increment of intensity caused by heating is varied along the droplet surface. The intensity increment near the edge area is larger than it at the apex area, which indicates that the higher substrate temperate enhance the THF evaporation on the droplet surface layer, especially near the contact line area. Additionally, p-AIEgen is selected for this work due to the following characteristics: a. the increasing fluorescence intensity is in a monotonic fashion upon the water adding; b. p-AIEgen is suitable in a range of particle size that is small enough to be evenly dispersed in the droplet; c. This p-AIEgen shows good photo-stability upon continuous laser excitation, which is up to 14mW, during the first and second evaporation stage without any fluorescence intensity quenched. The method of using AIEgens to visualize the concentration distribution can be used for different binary water/solvent systems. However, we may need to select different AIEgens for different systems to optimized measurement. We will continue to explore further in alcohol/water system by utilizing different AIEgens.


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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions X.C. and H.Q. conceived and designed the idea. X.C. and N.X. performed the material selection and droplet evaporation experiments. Z.Q. and N.X. and B.T. performed synthesis of the material, dilute solution and samples of p-AIEgen. X.C. and J.Y. performed the video processing of the experiments. K.W. helped with the setup of laser system. X.C., M.H. and H.Q. discussed and interpreted the results. The manuscript was written using contributions from all authors.

ACKNOWLEDGMENT This work was supported by the Research Grants Council (RGC/GRF Project No.: 16207515) of Hong Kong Government.

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Aggregation-Induced Emission Luminogen-Based Direct Visualization

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