Direct Visualization of the Behavior and Shapes of the Nanoscale

Subscriber access provided by UNIV AUTONOMA DE COAHUILA UADEC

Biological and Environmental Phenomena at the Interface

Direct visualization of the behavior and shapes of the nanoscale menisci of an evaporating water droplet on a hydrophilic nanotextured surface via high-resolution synchrotron X-ray imaging Dong In Yu, Seung Woo Doh, Ho Jae Kwak, Jiwoo Hong, Narayan Pandurang Sapkal, and Moo Hwan Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04109 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Direct visualization of the behavior and shapes of the nanoscale menisci of an evaporating water droplet on a hydrophilic nanotextured surface via high-resolution synchrotron X-ray imaging Dong In Yu1, Seungwoo Doh2, Ho Jae Kwak3, Jiwoo Hong3, Narayan Pandurang Sapkal1, and Moo Hwan Kim,2,3* 1Department

of Mechanical Design Engineering, Pukyong National University, Busan, 48547, Republic of Korea

2Division

of Advanced Nuclear Engineering, POSTECH, Pohang, 37673, Republic of Korea

3Department

of Mechanical Engineering, POSTECH, Pohang, 37673, Republic of Korea

Corresponding Author: *Professor, Moo Hwan Kim Department of Mechanical Engineering, POSTECH, Pohang, Republic of Korea Tel: +82-54-279-8157 E-mail: [email protected] ACS Paragon Plus Environment

1

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

ABSTRACT

Despite the considerable research interest due to omnipresent and practical importance of interfacial phenomena (e.g., wetting and de-wetting) on nano-textured surfaces in the academic and industrial fields, directly visualizing the behavior and shapes of liquid-vapor interfaces between nano-scale structures remains an arduous task because of the resolution limitations of visualization techniques. In this study, we succeeded in a first-hand visualization of the behavior and shapes of liquid-vapor interfaces of a water droplet between nanometer-scale pillar during evaporation by introducing a synchrotron X-ray imaging technique with spatially high resolution (40nm/a pixel). On the basis of the visualization data, we intensively analyzed and discussed the spreading and evaporation phenomena of a liquid droplet on hydrophilic nano-textured surfaces.

KEYWORDS Wetting (spreading), evaporation, hydrophilic, nano, synchrotron X-ray imaging

ACS Paragon Plus Environment

2

Page 3 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

INTRODUCTION The transfer of mass and heat in multiphase systems, which coexist at three phases (gas, liquid and solid) is considerably influenced by wettability depending on the chemical or mechanical characteristics (e.g., material properties and roughness) of solid surfaces. This phenomenon is attributed to the transfer of mass and heat happening primarily with phase changes at interfaces between phases. By contrast, extremely wetting features, such as super-hydrophilic and super-hydrophobicity, are known to occur on nano-textured surfaces, thanks to the rapid advance of nanofabrication techniques. Such inherent wetting properties of nano-textured surfaces have been adopted in numerous engineering applications1-24, such as self-cleaning and anti-icing surface, drag reduction and enhanced heat transfer, because they contribute to enhancing the performances of such multiphase systems in terms of mass and heat transfer. To understand interfacial phenomena that occur on nano-textured surfaces, various experimental endeavors to visualize liquid-vapor interfaces have been reported25-30. To the best of our knowledge, however, nearly all these studies are restricted to visualizing the upper part of liquid-vapor interfaces on nano-textured surfaces through the use of visible rays. Given the optically intrinsic limitations of visible rays, such as a relatively low spatial resolution and refraction at interfaces between phases, directly visualizing liquid-vapor interfaces at nanometer scales on nano-textured surfaces is extremely challenging. For this reason, information on liquid-vapor interfaces with nanometer scales should be indirectly inferred through interfacial imaging experimentally visualized far from nano-textured surfaces. To alleviate technical limitations of visualization technique based on visible ray, interference imaging technique has been introduced in the research fields of small scaled interfacial phenomena31,32. In particular, on the basis of the imaging technique, understanding for evaporation phenomena of thin film (or sessile droplet) has been significantly concreted by outstanding researchers33-35. However, visualizing liquid-vapor interfaces at nanometer scales on nano-textured surfaces is still challenging, since the technique has been applied limitedly to visualize interfaces on flat and transparent solid surfaces. The mass and heat transfer phenomena in multiphase systems arise dominantly at the three phase contact line and the liquid-vapor interfaces. Thus, the direct visualization of liquid-vapor interfaces with nanometer scales is necessary to understanding the underlying interfacial phenomena that occur on nano-textured surfaces and to advance engineering application in multiphase systems.


3

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 39

Here, we directly visualize the behavior and shapes of the meniscus of the liquid-vapor interfaces between the nanometer scale pillars of a water droplet during evaporation using high-resolution X-ray imaging. The synchrotron X-ray imaging technique allows for visualization with high spatial resolution and for elimination (or reduction) of image distortion at the interfaces. By utilizing the visualization technique, we can intuitively and clearly observe shapes and dynamic behaviors of liquid interfaces on nano-textured surfaces that are hard to visualize by conventional visible ray-based imaging techniques. A nano-textured and hydrophilic surface with a penetration depth of a synchrotron X-ray beam (~30 μm) is preferentially fabrication using photolithography and subsequent chemical etching processes. Then, the behavior and shapes of the liquid-vapor interfaces between the nano-pillars of an evaporating water droplet on the prepared surface are visualized via synchrotron X-ray imaging with high spatial resolution (40nm/a pixel) for side view observation. The consecutive top-view images of the liquid-vapor interfaces on nano-textured surfaces are also acquired via conventional visible ray imaging techniques using high-speed camera (~1000 fps) and a microscope (5×magnification). We comprehensively analyze the wetting and evaporation phenomena of sessile droplets on hydrophilic nano-textured surfaces on the basis of the visualized data.

EXPERIMENTAL SETUP Preparation of test sections In this study, we introduce the synchrotron X-ray beam (7C) at Pohang Acceleration Laboratory with a high spatial resolution (40 nm/a pixel) to visualize the liquid-vapor interfaces between nanometer-scale pillars. This beam line has a micro-scale penetration depth (~30 μm) that considers the attenuation rate of silicon materials. As shown in Fig. 1(a), we develop the test section with a spoon shape that consists of a reservoir for a water droplet and a liquid film pathway with a width 30 μm. Using a 4 inch double-polished silicon wafer, for masking during the silicon etching process, an aluminum layer with 3000 Å thickness is deposited using an e-beam evaporator. An aluminum mask with a spoon shape is patterned on the surface via photolithography and wet aluminum etching methods. A spoon shape with a height of 100 μm is fabricated on the surface using deep reactive ion etching (DRIE) processes. The deposited fluorine passivation during the DRIE processes and the remaining aluminum mask are removed via O2 plasma (600 W, 1 h) cleaning and the wet aluminum etching method, respectively. Nanometer-scale pillars with random arrays on the surface are fabricated using specific DRIE ACS Paragon Plus Environment

4

Page 5 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

processes36. The nanometer-scale pillars on the surface have a needle shape, as shown in Fig. 1(b). The nanotextured surfaces are called “black silicon” due to their high light absorption. They are currently applied in various research fields, such as imaging devices, laser photonics, and solar cells. The geometric morphology of black silicon is measured via synchrotron X-ray tomography37. The detailed geometry of the nanostructures in the test section is provided in Table. 1. The specific fabrication and cleaning processes are illustrated in Fig. 2. Visualization of liquid-vapor interfaces In this study, the behavior and shape of an evaporated droplet on a nano-textured surface are directly visualized from the side view via synchrotron X-ray imaging, as shown in Fig.3. A test section is initially positioned on the optic table using alignment mounts to adjust the focus. For fine focus, alignment mounts in which at test section can be moved with nanometer scales are used. In the well-positioned test section, a 2.5μL distilled ionized water droplet is gently placed on the syringe pump. Consequently, the behavior and shapes of the liquid–vapor interfaces between the nano-pillars in the test section are visualized via synchrotron X-ray imaging. Note that all the experiments with a X-ray beam source are conducted in a shielding room, and the experimental processes are controlled remotely from outside in order to protect the safety and health of the researchers. As you expected, a liquid droplet spreads as soon as it is placed on hydrophilic nano-textured surfaces. The spreading phenomenon occurs in a flash and thus visualizing it with the only synchrotron X-ray beam is not easy due to its time resolution limitation (~1fps) of the visualization technique. To compensate for the time resolution of synchrotron X-ray imaging, the behavior and shapes of the liquid–vapor interfaces on the prepared surface are visualized through additional experiments with a high-speed camera (~1000 fps) and an optical microscope (5×magnification) from the top view while a liquid droplet spreads, as shown in Fig.4.

VISUALIZATION AND ANALYSIS Overall phenomena on nano-textured surface As shown in Fig. 3, given the multiphase system of a water droplet on the hydrophilic nano-textured surface, the overall phenomenon of the system can be divided approximately into the spreading and evaporation processes for concise analysis. During the spreading process, a water droplet fills in a reservoir in the test section. A contact line of a water droplet is pinned to the edge of the reservoir, and a precursor of a water droplet gradually spreads in the liquid film pathway. The precursor of a droplet spreads maximally in the test sections when the multiphase ACS Paragon Plus Environment

5

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 39

system reaches an equilibrium condition. Subsequently, the volume of a water droplet on the surface decreases due to evaporation. During the evaporation process, the contact angle of a water droplet gradually decreases at the edge of the reservoir. When the contact angle reaches a receding contact angle, a pinned contact line of a water droplet is released at the edge of a reservoir in the test section. After a sufficient evaporation time, the bulk of a water droplet evaporates completely, but a volume of water droplet remains only in the spaces between the nanometer-scale pillars on the reservoir and liquid film pathway in a test section. Finally, a water droplet dries from outside the liquid film pathway to the center of the reservoir in the test sections. Spreading phenomena on nano-textured surface During the spreading process, we visualize the liquid–vapor interfaces in the test section from the top view using a high-speed camera and an optical microscope. When a spreading droplet in the test section is divided into the bulk and the precursor, the precursor line and the contact line of the bulk spread with different velocities, as shown in Fig. 4. The spreading process was reported to be mainly divided into two stages, inertia and contact line friction-dominated stages, in previous researches27, 28, In the first stage, a radius of the precursor and the bulk spreads monotonically with the equivalent velocity as time increases (~t). During this stage, the interfacial motions of a liquid droplet are dominantly influenced by inertial forces. Therefore, this stage is called the inertial regime27,38. In the second stage, the difference of the radius between the precursor line and the contact line of the bulk increases, and the radius of the precursor and the bulk spreads with root proportionality as time increases (~tn). During this stage, the interfacial motions of the bulk rapidly slow down, however the interfacial motion of precursor spreads ahead of the contact line of the bulk due to capillary force in the space between nano-pillars. At the precursor, an interfacial motion is generated by the capillary force in the space between the nano-pillars and prevented by the viscous force in the wetted area of the nano-pillars. As the space between nano-pillars decreases on the equivalent projected area of nano-textured surfaces, the capillary and viscous forces of the precursor increase simultaneously as shown in Eqs. (1) and (2)39.

Fcapillary ~ 2 lv

Fviscous ~

rh p2

(1)

lV p hR p

p 2 ln  p r 

(2)


6

Page 7 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

where Fcapillary, Fviscous, σlv, μl, h, r, p, Vp, and Rp are the capillary force, viscous force, surface tension of the liquid– vapor interfaces, viscosity of a liquid, height of nano-pillars, radius of nano-pillars, pitch between nano-pillars, velocity of a precursor line, and spreading radius of a precursor, respectively. A precursor spreads diffusively and is governed by the balance between two forces due to the geometric morphologies of nano-pillars25–30, 38-40. From the side view, we visualize the liquid–vapor interfaces in a test section via synchrotron X-ray imaging. The liquid–vapor interfaces between nano-pillars are visualized at the end of the spreading process and during the evaporation process, because visualizing fast interfacial phenomena during the spreading process is difficult using our visualization technique. Alternatively, we observe the thickness of the precursor in the test section at the end of the spreading process. In the previous work25, the liquid–vapor interfaces of the precursor are visualized from the top view. The local thickness of the precursor could be estimated from the differences in intensity of visible ray images. However, the thickness of the precursor in the synchrotron X-ray images varies in the longitudinal direction of a liquid film pathway in a test section as shown in Fig. 5. Accordingly, given the limitation in the fields of view, we visualize liquid–vapor interfaces in six different sections with small time intervals (from the end of the precursor line to the intersection between the reservoir and the liquid film channel) while the fields of view are moved manually using the remote control for the alignment mounts. When the orders of time to visualize different sections are considered, we determine that the thickness of the precursor is locally non-uniform on a nano-textured surface. Previous research25 showed that micrometer- and nanometer-scale pillars based on DRIE processes have contiuous scallop shapes at the side of the strcutures. Moreover, it was insisted that these shapes induce energy barriers and lead to a disruption of the liquie-vapor interfacial motions between pillars during the spreading phenomenon, thereby resulting in the locally non-uniform thickness of the precursor25. On the basis or the spreading model in the previous research25, non-uniform thickness during spreading process are well generated as scallop shape of nano-pillars is increases and spacing between nano-pillars is decreases. In this study, these scallop shapes are also visualized at the side of the nano-pillars of the test section as shown in Fig. 1(b). Comparing between test section in this study and test section in previous research, although scallop shapes of nano-pillars in this study is unclear than those in the previous research, it is shown that the nano-pillars in this study is more densely and randomly arrayed than those in previous research. Considering the previous research, it was shown that the locally varing thickness of the precursor are caused by the scallop shapes at the side of the nanometer-scale structures. ACS Paragon Plus Environment

7

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 39

Evaporation phenomena on nano-textured surface When an external flow and temperature difference between phases are absent, evaporation of a liquid droplet in air medium dominantly occurs through mass diffusision caused by difference in vapor concentration between the liquid–vapor interfaces and the environment, as shown in Eq. 3. An evaporation flux is uniform along the liquid– vapor interfaces41:

m&  4 Rd Ddiff  Cinterface  Cenvironment 

(3)

where m, Rd, Ddiff, and C refer to the evaporation rate, liquid droplet radius, diffusion coefficient, and vapor concentration, respectively. When a liquid droplet on solid surfaces is considered, interfaces exist between phases and the contact line. In such case, an evaporation flux along the liquid–vapor interfaces is intensively influenced by the shape of interfaces42, 43. The potential fields of vapor concentrations for an evaporated droplet on a solid surface can be generally analyzed in toroidal form because of singularities at the contact line. When a liquid droplet has an acute contact angle (θ90°) on hydrophobic surfaces, an evaporation flux is uniform along liquid–vapor interfaces44. On hydrophilic structured surfaces, a liquid droplet maintains an initial contact radius with the surfaces during most of the evaporation period. The evaporation mode is called “constant contact radius (CCR)”45, 46. Previous research indicates that CCR is caused by the pinning force at the contact line47 or the thermodynamic process rapidly reaching an equilibrium state during droplet evaporation48. In CCR evaporation mode, an equivalent liquid volume with non-uniform evaporated volume along the liquid–vapor interfaces should be supplied to a droplet to maintain its contact radius. Therefore, the inner flows of a droplet are generated from the center (or bulk) of the droplet to the contact (or precursor) line. Such flow is called “capillary-induced flow.” As shown in Eq. 4 and Fig. 6(a), in accordance with previous research44, 49, it has been analyzed theoretically and numerically that the inner radial flows of an evaporated droplet are proportional to the evaporation flux.

v  r  ~ j  r  ~ ( Rc  r )   ,  =   2   2  2 

(4)

where v, j, and Rc denote the velocity of capillary-induced flow, the evaporation flux, and the contact radius of a liquid droplet, respectively. In the test section, a droplet maintains CCR mode during most of the evaporation ACS Paragon Plus Environment

8

Page 9 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

period. As shown in Fig. 6(b) and Video. 150, the liquid–vapor interfaces between nano-pillars fluctuate during an evaporation process. General knowledge about droplet evaporation and the visualization data indicate that fluctuations of the liquid–vapor interfaces at the precursor are caused by capillary-induced flows due to the CCR evaporation mode and non-uniform evaporation flux along the liquid–vapor interfaces. In this study, it is insisted that the fluctuation of liquid-vapor interfaces between nano-pillars is intuitive and dominant evidence of the capillary-induced flow. Fig. 7 illustrates the behavior of the liquid–vapor interfaces of the precursor at the end of the evaporation process. As aforementioned in previous section, the thickness of a precursor on nano-textured surfaces is locally different in the radial direction due to the scallop shapes at the side of the nano-pillars. Geometrically, considering with an equivalent precursor volume and maximum liquid film thickness, the radius of a precursor with non-uniform thickness is larger than the radius of a precursor with uniform thickness, as shown in the comparison between Figs. 8(a) and 8(b). The total perimeter of the precursor line is larger when the precursor exhibits non-uniform thickness. Evaporation is generated considerably near the contact line, such that the non-uniform thickness of the precursor enhances the total evaporation rate on nano-textured surfaces. In addition, considering the needle (or circular cone) shape of black silicon, the perimeters of the contact lines hanging at nano-pillars increase when a precursor exhibits non-uniform thickness. Thus, the non-uniform thickness of a precursor enhances the total evaporation rate on nano-textured surfaces. A water droplet dries out from outside a liquid film channel to the center of the reservoir in the test sections because of the distribution of the evaporation flux in a precursor with uniform thickness.

CONCLUSION In this study, the liquid–vapor interfaces of a liquid droplet are visualized on a hydrophilic nano-textured surface to understand nanometer-scale interfacial phenomena. Through synchrotron X-ray imaging with high spatial resolution (40nm/pixel), the liquid–vapor interfaces between nanometer-scale pillars are visualized during the evaporation process. Considering technical limitations of synchrotron X-ray imaging such as low penetration depth of beam line (~30μm) and relatively low time resolution (~1fps), test section with spoon shape are fabricated by MEMS technique and the evolution of liquid–vapor interfaces during the spreading process in the test section are separately visualized with a high-speed camera and an optical microscope. Through the ACS Paragon Plus Environment

9

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 39

synchrotron X-ray images, we clearly visualize liquid films with non-uniform thickness at the precursor and capillary-induced flows of an evaporated droplet on a hydrophilic nano-textured surface. On the basis of the visualization data, we conclude that the inherent scallop shapes at the side of the nano-pillars cause the nonuniform thickness of the precursor, which enhances total evaporation rate on hydrophilic nano-textured surfaces. The present synchrotron X-ray imaging technique can directly visualize behaviors and shapes of the nanoscale menisci. Accordingly, it can provide us physical insights to uncover novel and interesting interfacial phenomena in multiphase systems with submicron scales (e.g., wetting and pool boiling). However, for the application of broad research and engineering fields, the synchrotron X-ray imaging has still technical limitations such as the limitedly geometrical features of test sections (configuration, dimension and so on), difficulties of 3 dimensional image realization, the relatively limited time resolution and so on. Recently, numerical studies based on Monte Carlo and molecular dynamics simulations have been widely carried out to give an intuition in the physical interpretation of nanometer scaled interfacial phenomena51-53, instead of technically limited experiments. We will find intuitive relation between computational prediction and experimental results with the assistance of molecular dynamic simulation in the near future work.

ACKNOWLEGMENT This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MOE) (NRF-2018R1D1A1B07048332). This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20184010201700). Experiments at the Pohang Light Sources were supported in part by MOE and POSTECH. We would like to thank J. Lim and S. Lee in PAL for conducting the experiments with the synchrotron X-ray beam lines (7C)

CONFLICT OF INTEREST The authors declare no competing financial interest.

ASSOCIATED CONTENT : Supperting Information


10

Page 11 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The Supporting Information is available free of charge on the ACS Publications website at DOI: The fluctuation of the liquid-vapor interfaces near the precursor line: The video is made by X-ray images. These images are obtained at near regions of a precursor each second. In this movie, the fluctuation of liquid-vapor interfaces between nano-pillars is the dominant evidence of the capillary induced flow during droplet evaporation. (AVI)


11

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 39

REFERENCES (1)

A. V. Yakovlev, V. A. Milichko, V. V. Vinogradov and A. V. Vinogradov, Inkjet color printing by interference nanostrcutures, ACS Nano, 2016, 10, 3078-3086.

(2)

C. Check, R. Chartoff and S. Chang, Inkjet printing of 3D-composites formed by photopolymerization of an acrylate monomer, React. Funct. Polym. 2015, 97, 116-122.

(3)

H. Nam, K.. Song, D. Ha and T. Kim, Inkjet printing based mono-layered photonic crystal patterning for anti-counterfeiting structural colors, Sci. Rep, 2016, 6, 30885.

(4)

Q. Zhou, A. K. Thokchom, D. –J. Kim, T. Kim, Inket-printed Ag micro-/nanostrcutrure clusters on Cu substrates for in-situ pre-concentration and surface-enhanced Raman scattering, Sens. Actuator B Chem. 2017, 243, 176-183.

(5)

A. Nakajima, K. Hashitomo, T. Watanabe, K. Taki, G. Yamaguchi and A. Fujishima, Transparent superhydrophobic thin films with self-cleaning properties, Langmuir, 2000, 16, 7044-7047.

(6)

R. Fürstner, W. Barthlott, C. Neinhuis, P. Walzel, Wetting and self-cleaning properties of artificial superhydrophobic surfaces, Langmuir, 2005, 21, 956-961.

(7)

Y. Su, B. Ji, K. Zhang, H. Gao, Y. Huang and K. Hwang, Nano to micro structural hierarchy is crucial for stable superhydrophobic and water-repellent surfaces, Langmuir, 2010, 26, 4984-4989.

(8)

T. Onda, S. Shibuichi, N. Satoh and K. Tsujii, Super-water-repellent fractal surfaces, Langmuir, 1996, 12, 2125-2127

(9)

K. Akbar, J. H. Kim, Z. Lee, M. Kim, Y. Yi and S. –H. Chun, Superaerophobic graphene nano-hills for direct hydrazine fuel cells, NPG Asia Mater. 2017, 9, e378

(10)

A. Serov, K. Artyushkova, E. Niangar, C. Wang, N. Dale, F. Jaouen, M. –T. Sougrati, Q. Jia, S. Mukerjee and P. Atanassov, Nano-structured non-platinum catalysts for automotive fuel cell application, Nano energy, 2015, 16, 293-300.

(11)

J. Heinemann, K. Deng, S. C. C. Shih, J. Gao, P. D. Adams, A. K. Singh and T. R. Northen, On-chip integration of droplet microfluidics and nanostructure-initiator mass spectrometry for enzyme screening, Lab Chip, 2017, 17, 323-331.

(12)

E. Angeli, C. Manneschi, L. Repetto, G. Firpo and U. Valbusa, DNA manipulation with elastomeric nanostructures fabricated by soft-moulding of a FIB-patterned stamp, Lab Chip, 2011, 11, 2625-2629 ACS Paragon Plus Environment

12

Page 13 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(13)

Langmuir

J. –J. Kim, Y. Lee, H. G. Kim, K. –J. Choi, H. –S, Kweon, S. Park and K. –H. Jeong, Biologially inspired LED lens from cuticular nanostructures of firefly lantern, PNAS, 2012, 109, 18674-18678.

(14)

A. Horrer, J. Haas, K. Freudenberger, G. Gauglitz, D. P. Kern and M. Fleischer, Compact pasmonic optical biosensors based on nanostructured gradient index lenses integrated into microfluidic cells, Nanoscale, 2017, 9, 17378-17386.

(15)

S. Fischer, R. P. Sahu, S. Sinha–Ray, A. L. Yarin, T. Gambaryan-Roisman and P. Stephan, Effect of nanotextured heater surfaces on evaporation at a single meniscus, Int. J. Heat Mass Trans, 2017, 108, 24442450.

(16)

H. Kim and J. Kim, Evaporation characteristics of a hydrophilic surface with micro-scale and/or nanoscale structures fabricated by sandblasting and aluminum anodization, J. Micromech. Microeng., 2010, 10, 045008.

(17)

Z. Lu, S. Narayanan and E. N. Wang, Modeling of evaporation from nanopores with nonequilibrium and nonlocal effects, Langmuir, 2015, 31, 9817-9824.

(18)

D. Brutin and V. Starov, Recent advances in droplet wetting and evaporation, Chem. Soc. Rev, 2018, 47, 558-585.

(19)

D. H. Shin, S. H. Lee, C. K. Choi and S. Retterer, The evaporation and wetting dynamics of sessile droplets on submicron-scale patterned silicon hydrophobic surfaces, J. Micromech. Microeng. 2010, 20, 055021.

(20)

A. Zou, S. C. Maroo, Critical height of micro/nano structures for pool boiling heat transfer enhancement. Appl. Phys. Lett. 2013, 103, 221602.

(21)

M. M. Rahman, E. Ölçeroğlu and M. McKarthy, Role of wickability on the critical heat flux of structured superhydrophilic surfaces, Langmuir, 2014, 30, 11225-11234.

(22)

H. D. Kim and M. H. Kim, Effect of nanoparticle deposition on capillary wicking that influences the critical heat flux in nanofluids. Appl. Phys. Lett., 2007, 91, 014104.

(23)

K. –H. Chu, Y. S. Joung, R. Enright, C. R. Buie and E. N. Wang, Hierarchically structured surfaces for boiling critical heat flux enhancement, Appl. Phys. Lett., 2013, 102, 151602.

(24)

N. S. Dhillon, J. Buongiorno and K. K. Varanasi, Critical heat flux maxima during boiling crisis on textured surfaces. Nat. Commun., 2015, 6, 8247. ACS Paragon Plus Environment

13

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(25)

Page 14 of 39

R. Xiao, K. –H. Chu and E. N. Wang, Multilayer liquid spreading on superhydrophilic nanostructured surfaces, Appl. Phys. Lett., 2009, 94, 193104.

(26)

R. Xiao, R. Enright and E. N. Wang, Prediction and optimization of liquid propagation in micropillar arrays, Langmuir, 2010, 26, 15070-15075.

(27)

H. S. Ahn, G. Park, J. Kim and M. H. Kim, Wicking and spreading of water droplets on nanotubes, Langmuir, 2012, 28, 2614-2619.

(28)

X. Chen, J. Chen, X. Ouyang, Y. Song, R. Xu and P. Jiang, Water droplet spreading and wicking on nanostructured surfaces, Langmuir, 2017, 33, 6701-6707.

(29)

C. K. Wemp and V. P. Carey, Water wicking and droplet spreading on randomly structured thin nanoporous layers, Langmuir, 2017, 33, 14513-14525.

(30)

W. Yang and J. Xu, Drop spreading and penetrating on micro/nano particle sintering porous with multiscale structure, Colloid Surf. A Physicochem. Eng. Asp., 2017, 515, 9-22.

(31)

S. G. Subramanian, M. Chakraborty, S. Tenneti and S. DasGupta, Electrodewetting and Wetting of an Extended Meniscus, Langmuir, 2018, 34, 9897-9906.

(32)

E. Y. Gatapova, A. M. Shonina, A. I. Safonov, V. S. Sulyaeva and O. A. Kabov, Evaporation dynamics of a sessile droplet on glass surfaces with fluoropolymer coatings: focusing on the final stage of thin droplet evaporation, Soft Matter, 2018, 14, 1811-1821.

(33)

T. T. T. Nguyen, J. Yu, J. L. Plawsky, P. C. Wayner Jr, D. F. Chao and R. J. Sicker, Spontaneously oscillating menisci: Maximizing evaporative heat transfer by inducing condensation, Int. J. Therm. Sci., 2018, 128, 137-148.

(34)

A. Kundan, T. T. T. Nguyen, J. L. Plawsky, P. C. Wayner Jr, D. F. Chao and R. J. Sicker, Condensation on highly superheated surfaces: unstable thin films in a wickless heat pipe, Phys. Rev. Lett., 2017, 118, 094501.

(35)

T. T. T. Nguyen, A. Kundan, P. C. Wayner Jr, J. L. Plawsky, D. F. Chao and R. J. Sicker, The effect of an ideal fluid mixture on the evaporator performance of a heat pope in microgravity, Int. J. Heat and Mass Trans., 2016, 95, 765-772.


14

Page 15 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(36)

Langmuir

H. Jansen, M. de Boer, R. Legtenberg and M. Elwenspoek, The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control, J. Micromech. Microeng., 1995, 5, 115-120.

(37)

D. I. Yu, H. J. Kwak, C. M. Park, C. Choi and M. H. Kim, The wetting criteria of the intrinsic contact angle between hydrophilic and hydrophobic micro/nano-textured surfaces: experimental and theoretical analysis with the synchrotron X-ray imaging, Langmuir, 2019, 35(10), 3607-3614.

(38)

D. Quere, Inertial capillarity, Europhys. Lett., 1997, 39, 533-538.

(39)

C. Ishino, M. Reyssat, E. Reyssat, K. Okumura and D. Quere, Wicking within forests of micropillar, Europhys. Lett., 2007, 79, 56005.

(40)

J. Li, X. Zhou, J. Li, L. Che, J. Yao, G. McHale, M. K. Chaudhury and Z. Wang, Topological liquid diode, Sci. Adv., 2017, 3, 3530.

(41)

I. Langmuir, The evaporation of small sphere, Physical review, 1918, 12, 368-370.

(42)

K. S. Birdi, D. T. Yu and A. Winter, A study of the evaporation rates of small water drops placed on a solid surface, J. Phys. Chem., 1989, 93, 3702-3703.

(43)

K. S. Birdi and D. T. Yu, Wettability and the evaporation rates of fluids from solid surfaces, J. Adhes. Sci. Technol., 1993, 7, 485-493.

(44)

T. A. H. Nguyen, A. V. Nguyen, M. A. Hampton, Z. P. Xu, L. Huang and V. Rudolph, Theoretical and experimental analysis of droplet evaporation on solid surfaces, Chem. Eng. Sci., 2012, 69, 522-529.

(45)

R. G. Picknett and R. Bexon, The evaporation of sessile or pendent drops in still air, J. Colloid Interface Sci., 1977, 64, 336-350.

(46)

H. Y. Erbil, G. McHale, M. I. Newton, Drop evaporation on solid surfaces: Constant contact angle mode, Langmuir, 2002, 18, 2636-2641.

(47)

M. E. R. Shanahan, Simple theory of “stick-slip” wetting hysteresis, Langmuir, 1995, 11, 1041-1043

(48)

D. I. Yu, H. J. Kwak, S. Doh, H. S. Ahn, H. S. Park, K. Moriyama and M. H. Kim, Dynamics of contact line depinning during droplet evaporation based on thermodynamics, Langmuir, 2015, 31, 1950-1957.

(49)

R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel and T. A. Witten, Capillary flow as the cause of ring strains from dried liquid droplet, Nature, 1997, 389, 827-829.

(50)

See Supporting Information ACS Paragon Plus Environment

15

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(51)

Page 16 of 39

F. H. Song, B. Q. Li and C. Liu, Molecular dynamics simulation of the electrically induced spreading of an ionically conducting water droplet, Langmuir, 2014, 30, 2394-2400.

(52)

F. H. Song, B. Q. Li and C. Liu, Molecular dynamics simulation of nanosized water droplet spreading in an electri field, Langmuir, 2013, 39, 4266-4274.

(53)

R. Holyst, M. Litniewski and D. Jakubczyk, Evaporation of liquid droplets of nano- and micro-meter size as a function of molecular mass and intermolecular interactions: experiments and molecular dynamics simulations, Soft Matter, 2017, 13, 5838-5864.


16

Page 17 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table of Contents List of tables Table.1 Specific geometrical morphologies of nanometer scaled structures on test section

List of the figure Figure 1. Test section (a) photograph (b) SEM images (reservoir, liquid film pathway, scallop shapes at the side area of nanometer scaled structures) Figure 2. Specific fabrication and cleaning processes to prepare the test section Figure 3. Experimental apparatus and overall phenomena on the test sections Figure 4. The different spreading velocity between the contact line of the bulk and the precursor line (‘t0-’ is a time before a liquid droplet contacts initially on a test. ‘t0+’ is difference of the time between time when a liquid droplet contacts initially on a test section and time when a precursor line is captured by high speed camera (in a field of view)) (a) Visualization with the high speed camera and microscope (images), (b) temporal evolutions of spreading behaviors of the contact line of the bulk and the precursor line. Figure 5. The non-uniform thickness of the precursor on the nano-textured surface (Visualization with the synchrotron X-ray imaging) Figure 6. Non-uniform evaporation flux and the capillary induced flow between the nano-structures (a) Schematics of the evaporation flux along the liquid-vapor interface on the hydrophilic surface (b) The fluctuation of the liquid-vapor interfaces near the precursor line (Visualization with the synchrotron X-ray imaging) Figure 7. The behavior of the liquid-vapor interfaces of the precursor at the end of the evaporation process (Visualization with the synchrotron X-ray imaging) Figure 8. Schematics of the enhancement mechanism of the evaporation process on the nano-textured surfaces. The shape of the contact line at the precursor (a) with the uniform thickness (b) with the non-uniform thickness

Graphical Abstract

List of the video


17

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 39

Video 1. The fluctuation of the liquid-vapor interfaces near the precursor line: The video is made by X-ray images. These images are obtained at near regions of a precursor each second. In this movie, the fluctuation of liquid-vapor interfaces between nano-pillars is the dominant evidence of the capillary induced flow during droplet evaporation.


18

Page 19 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table.1 Specific geometrical morphologies of nanometer scaled structures on test section Parameters

Value

Roughness ratio on the basis of Wenzel state, fW

16.67

Roughness ratio on the basis of Cassie-Baxter state, fC-B

0.114


19

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 39

Figure 1. Test section (a) photograph (b) SEM images (reservoir, liquid film way, scallop shapes at the side area of nanometer scaled structures)


20

Page 21 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. Specific fabrication and cleaning processes to prepare the test section


21

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 39

Figure 3. Experimental apparatus and overall phenomena on the test sections


22

Page 23 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 4. The different spreading velocity between the contact line of the bulk and the precursor line (‘t0-’ is a time before a liquid droplet contacts initially on a test. ‘t0+’ is difference of the time between time when a liquid droplet contacts initially on a test section and time when a precursor line is captured by high speed camera (in a field of view)) (a) Visualization with the high speed camera and microscope (images), (b) temporal evolutions of spreading behaviors of the contact line of the bulk and the precursor line.


23

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39


24

Page 25 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 5. The non-uniform thickness of the precursor on the nano-textured surface (Visualization with the synchrotron X-ray imaging)


25

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 39

Figure 6. Non-uniform evaporation flux and the capillary induced flow between the nano-structures (a) Schematics of the evaporation flux along the liquid-vapor interface on the hydrophilic surface (b) The fluctuation of the liquid-vapor interfaces near the precursor line (Visualization with the synchrotron X-ray imaging)


26

Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 7. The behavior of the liquid-vapor interfaces of the precursor at the end of the evaporation process (Visualization with the synchrotron X-ray imaging)


27

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

Figure 8. Schematics of the enhancement mechanism of the evaporation process on the nano-textured surfaces. The shape of the contact line at the precursor (a) with the uniform thickness (b) with the non-uniform thickness


28

Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Graphical Abstract


29

Cover art 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Langmuir

Page 30 of 39

Visualization Nano-textured surface

Liquid-vapor interfaces of the precursor at the side view (Synchrotron X-ray imaging)


Page 31 of 39 Figure.1, Test section (a) photograph (b)Langmuir SEM images (reservoir, liquid film way, scallop shape at the side area of the nano-structures) 1 2 3 4 5 Reservoir 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26Liquid film 27 pathway 28 29(length:20mm, 30 31 width:30μm) 32 33 34 35 36 10μm 37 38 39 40 41

(a)

(b)

(radius:3mm)

Spoon shape (height: 100μm)

10μm

1μm ACS Paragon Plus Environment

Side view (scallop shapes)

Langmuir Figure.2, Specific fabrication and cleaning process to prepare the test section 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

4” Silicon wafer

Aluminum masking removing & piranha cleaning

Aluminum 3000Å (E-beam evaporator)

O2 cleaning (1000W, 30min)

Aluminum masking (lithography + wet etching)

Spoon shape etching (100μm) (DRIE)

Black silicon etching (6μm) (DRIE)

O2 cleaning (1000W, 30min)


Page 32 of 39

Page 33 of 39 Langmuir phenomena on the test sections Figure.3, Experimental apparatus and overall 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

CCD Camera

CPU (Automatic control)

Scintillator & mirror

Shielding room

Test section

Syringe pump

Alignment mount

Synchrotron Shutter X-ray beam

Optic table

①

②

④

③

visualization


Spreading process (visible ray)

⑤

visualization

visualization

Evaporation process (X-ray)

34 of 39 Figure.4, The different spreading velocityLangmuir between the contact line of the bulk Page and the precursor line (‘t0-’ is a time before a liquid droplet contacts initially on a test. ‘t0+’ 1 is 2 difference of the time between time when a liquid droplet contacts initially on a 3 test section and time when a precursor line is captured by high speed camera (in a 4 5 field of view)) (a) Visualization with the high speed camera and microscope (images), 6 7 (b) temporal evolutions of spreading behaviors of the contact line of the bulk and 8 9 the precursor line. 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(a) reservoir

liquid film pathway

t=t0-

50μm precursor line on the reservoir

t=t0++10ms precursor line on the liquid film pathway contact line of the bulk

t=t0++39ms t=t0++175ms


t=t0++219ms

(b)

Page 35 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Langmuir


Langmuir Page 36 of 39 Figure.5, The non-uniform thickness of the precursor at the nano-textured surface (synchrotron X-ray images) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Reservoir

Liquid film pathway (width: 30μm) t=t0 (Beginning the evaporation process)

5μm

5μm

t=t6

t=t5

t=t4

t=t3

t=t4

t=t6


t=t2

t=t1

t=t1

Page 37 of 39 Langmuir Figure.6 Non-uniform evaporation flux and the capillary induced flow between the nano-structures (a) Schematics of the evaporation flux along the liquid-vapor 1 interfaces on the hydrophilic surface (b) The fluctuation of the liquid-vapor interfaces 2 3 near the precursor line (Visualization with the synchrotron X-ray imaging) 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(a)

C~const

∇C~const

J j(Rc)

rc

θ

Rc ②(t=t0+2s)

①(t=t0)

(b) δ~3.84μm 5μm

δ~2.90μm ACS Paragon Plus Environment

③(t=t0+5s)

δ~3.57μm

Langmuir Page the 38 of 39 Figure. 7 The behavior of the liquid-vapor interfaces between nano-structures at end of the evaporation process (The synchrotron X-ray imaging) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

①

④

②

③

⑤

⑥

5μm


Page 39 of 39 8 Schematics of the enhancement Langmuir Figure. mechanism of the evaporation process on the nano-textured surfaces. The shape of the contact line at the precursor (a) with 1 the uniform thickness (b) with the non-uniform thickness 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(a)

RP

RP

Precursor line

r uniform

Ρ uniform =2π r RP

(b)

RP

Contact line of the bulk

r

Ρ non-uniform =

non-uniform

θ Equivalent volume of the precursor


2π r cos θ

Direct Visualization of the Behavior and Shapes of the Nanoscale

Recommend Documents