Dynamics of thin precursor film in wetting of nanopatterned surfaces

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved August 4, 2021 (received for review April 29, 2021)
September 17, 2021
118 (38) e2108074118

Significance

Wetting of rough surfaces is ubiquitous in nature and plays an important role in many natural and industrial processes. However, not much is known about the nanoscale details of the wetting. This gap in our understanding of wetting stems from a lack of suitable methods that enable direct imaging of interactions between liquids and nanoscale patterns. Here, using liquid-phase transmission electron microscopy, we directly image the wetting of dense arrays of patterned vertical nanopillars. Our observations show that wetting proceeds through the spreading of an ultrathin precursor liquid film followed by complete capillary-action–induced wetting of the nanopillars. Our observations that shed light on the details of wetting are critical for numerous applications in nanofabrication and surface engineering.

Abstract

The spreading of a liquid droplet on flat surfaces is a well-understood phenomenon, but little is known about how liquids spread on a rough surface. When the surface roughness is of the nanoscopic length scale, the capillary forces dominate and the liquid droplet spreads by wetting the nanoscale textures that act as capillaries. Here, using a combination of advanced nanofabrication and liquid-phase transmission electron microscopy, we image the wetting of a surface patterned with a dense array of nanopillars of varying heights. Our real-time, high-speed observations reveal that water wets the surface in two stages: 1) an ultrathin precursor water film forms on the surface, and then 2) the capillary action by nanopillars pulls the water, increasing the overall thickness of water film. These direct nanoscale observations capture the previously elusive precursor film, which is a critical intermediate step in wetting of rough surfaces.
The wettability of surfaces plays an important role in many natural and industrial processes (13). Wetting of a smooth surface by a liquid is commonly described by the Young’s equation, which quantifies the wettability of a solid surface using the contact angle of a liquid droplet (4, 5). When the surfaces are rough, the spreading of a droplet is governed by contact angle and capillary effects, and the droplet spreads via hemiwicking (6, 7). The rough features of the surface act as capillaries, or wicks, that imbibe liquid from the droplet (810). Earlier studies show that modifying the surface roughness can be a powerful approach to tuning the surface wettability (1118), and such surface modifications have been used for applications in biomedicine (19, 20), textile industry (21, 22), and water treatment (23, 24).
Despite the recent progress in designing rough superhydrophilic surfaces (17, 24), the nanoscale details of the wicking process on rough surfaces remain unknown because these processes are extremely challenging to visualize. Current approaches to study wetting are based on optical methods (2528), and, while providing valuable insights, these methods lack the spatial resolution needed to discern the nanoscale details of wetting. Alternative approaches based on scanning probe microscopy techniques provide high spatial resolution but lack temporal resolution and thus have been limited to study the adsorption of water (29) and wetting of smooth surfaces (30, 31). Recent advances in liquid-phase transmission electron microscopy (TEM) (3236) and fast electron detection cameras (37, 38) enabled direct imaging of the nanoscale dynamics of liquids on surfaces with high temporal and spatial resolutions. Here, we used this liquid-phase TEM approach to study the wetting of patterned nanostructures at high speeds (200 to 300 frames per second [fps]).

Results and Discussion

Cylindrical Si nanopillars with heights of 230, 285, and 380 nm, diameter of 25 nm, and pitch of 90 nm were patterned on a SiNx film of a Si wafer from which the liquid cells were fabricated (Fig. 1) (3942). Prior to assembling the chips into a sealed liquid cell (Fig. 1A) in a custom-built flow holder (SI Appendix, section 1), the bottom chip with nanopillars was rendered hydrophilic using air-plasma treatment (SI Appendix, section 2). The measured contact angle (<10°) was sufficiently low for the wicking (SI Appendix, section 3).
Fig. 1.
Experimental setup used for in situ TEM imaging. (A) Schematic of a liquid cell comprising bottom and top Si chips with SiNx membrane windows used for real-time TEM imaging of the wetting of Si nanopillars. Si nanopillars are patterned on the window of a bottom chip. (B) Cross-sectional SEM and (C) top-down TEM images of 230-, 285-, and 380-nm–tall nanopillars. The diameters of all the nanopillars are approximately 25 nm (SI Appendix, Fig. S3).
In situ TEM time-series images in Fig. 2 show the wetting of nanopillar arrays with three different heights by wicking. The image series for the 230-nm nanopillars (Fig. 2A) shows that the SiNx membrane first partially wets by an ultrathin precursor water film (Fig. 2A, red line), and only then, the thicker water film, trailing the precursor film, starts to wet the nanopillars (Fig. 2A, yellow line).
Fig. 2.
Wetting of an array of Si nanopillars. (Top) Schematic and (Bottom) time-series of in situ TEM images showing the wetting of (A) 230-nm (Movie S1), (B) 285-nm (Movie S2), and (C) 380-nm (Movie S3) Si nanopillars. In all the cases, a precursor film forms first, followed by the wetting of nanopillars through capillary action. Because of the capillary forces, softer 285- and 380-nm–tall nanopillars bend to form clusters of 4 to 10 nanopillars, reducing the spacing between the nanopillars in these clusters and promoting further capillary pull of water into the clusters. After full wetting, the clustered nanopillars restore back to their original upright position. SI Appendix, Fig. S5 shows the corresponding false-colored images that estimate the water film thicknesses. Note that for the 380-nm–tall nanopillars, the length of the precursor film (i.e., the distance from the thick liquid to the edge of the precursor film) is dprecursor=2.2 µm, as it covers the entire FOV. t0 represents the last frame where the surface was still completely dry (i.e., before the onset of wetting) during imaging.
Movie S1.
Movie showing the wetting of the 230-nm-tall nanopillars described in Figure 2A and Figure S5A. Imaging was done at 300 frames per second (fps), and every three consecutive frames were averaged to improve the image contrast.
Movie S3.
Movie showing the wetting of the 380-nm-tall nanopillars described in Figure 2C and Figure S5C. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.
Movie S2.
Movie showing the wetting of the 285-nm-tall nanopillars described in Figure 2B and Figure S5B. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.
The ultrathin precursor water film also forms during the wetting of the 285- and 380-nm nanopillars. However, in contrast to the stiffer 230-nm nanopillars (see SI Appendix, section 10 for bending stiffness of different nanopillars), wetting of the softer 285- and 380-nm–tall nanopillars (Fig. 2 B and C) shows that after the formation of the precursor film, some nanopillars undergo capillary-induced bending and clustering. This clustering increases the capillary pull between the bent nanopillars (i.e., nanodroplets with a thicker water film form within the nanopillar clusters, which can be identified as small regions with a darker contrast). The clustered nanopillars restore to their upright position once the liquid thickness across the entire area increases to fully wet the patterned surface. Here, we mention that earlier studies describing the wetting of the patterned surfaces propose that the wicking front of the water film spreads by directly filling the patterned features on the surface (13, 27, 28, 43).
To better assess the role of capillary action on wetting, we tracked the evolution of water thickness with millisecond resolution. Fig. 3 shows the time-resolved details of the wicking process for all three types of nanopillars arrays. At tt0 = 0.00 s, the nanopillars were dry, and in less than 0.1 s, the precursor water film spreads over the entire SiNx surface in the field of view (FOV). For the 230-nm–tall nanopillars (Fig. 3 A and D), different regions wet differently. As the water fills the FOV to reach the height of 180 nm, the water thickness in a region with few missing nanopillars (Fig. 3A, lime-colored box) increases at a slower rate than at the rest of the patterned area. This behavior is consistent with a weaker capillary pull of the sparsely patterned area (i.e., wider capillary).
Fig. 3.
Dynamics of capillary pull during wetting. Time-series of in situ TEM images showing the wetting of (A) 230- (Movie S4), (B) 285- (Movie S5), and (C) 380-nm–tall (Movie S6) Si nanopillars. In all three cases, we observe an ultrathin precursor water film spanning the entire FOV. The stiff (shorter) 230-nm nanopillars remain intact, whereas many of the softer (taller) 285- and 380-nm nanopillars bend and cluster because of the capillary forces from nanodroplets. Eventually, as the wetting gets more pronounced and the overall thickness of the water film increases, the nanopillars in the clusters restore to their original upright position. The last image frame corresponds to bulk liquid reaching the FOV and blocking the electron beam. Plots of the average water thicknesses in the FOV (black lines) and in the small colored square regions for (D) 230- (lime), (E) 285- (blue and red), and (F) 380-nm–tall (orange and green) nanopillars. t0 represents the last frame where the surface was still completely dry during imaging. The dashed orange lines in D–F denote the corresponding nanopillar heights schematically illustrated on the right side of the plots.
Movie S4.
High-resolution movie showing the wetting of the 230-nm-tall nanopillars described in Figure 3A. Imaging was done at 200 fps, and every two consecutive frames were averaged to improve the contrast.
Movie S6.
High-resolution movie showing the wetting of the 380-nm-tall nanopillars described in Figure 3C. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.
Movie S5.
High-resolution movie showing the wetting of the 285-nm-tall nanopillars described in Figure 3B. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.
The image series for 285-nm (Fig. 3 B and E) and 380-nm (Fig. 3 C and F) nanopillars, which are softer and thus can easily bend, reveal that capillary-induced bending locally accelerates wetting through capillary action of clustered nanopillars. As the level of water nanodroplets within the clusters reaches 220 and 300 nm for 285- and 385-nm nanopillars, respectively, the nanodroplets cease to increase in height, while the water level outside the clusters continues to rise gradually. Once the thickness of the water outside the clusters reaches the same heights as the nanodroplets, most of the bent nanopillars restore back to their upright position, meaning that wetting does not cause any permanent adhesion between the clustered nanopillars (SI Appendix, section 11).
As mentioned above, earlier studies propose that a liquid droplet spreads on a rough surface by imbibition (SI Appendix, section 9), where the rough features act as capillaries, pulling the liquid from the droplet. Our observations show that an ultrathin precursor film precedes the wetting of nanoscopic rough hydrophilic surfaces, and the wetting is driven by an upward capillary pull on the ultrathin precursor film. Moreover, our observations distinctly reveal that the formation of a precursor water film is followed by the formation of nanodroplets arising from the capillary action of the nanopillar clusters (Figs. 2 B and C and 3 B and C). Also, the relatively slow filling of the regions with local defects with weaker capillary pull (Fig. 3A, lime box; B, red box; and C, orange box) confirms that, during wicking, the water thickness increases due to the capillary action.
To get robust statistics of the liquid film thickness at different stages of the wetting, we estimated the thickness of the precursor and final water films from multiple experiments. From Fig. 2, we note that the entire FOV may not wet at the same time, so each image in a given movie was divided into 64 (8 × 8) smaller regions, and the local water thickness (H) in these regions was estimated using the single scattering approximation (SI Appendix, section 5) (44):
H=λlnI0I.
[1]
Here, λ = 400 nm is the elastic mean free path of 200 keV electrons in water (45), I0 is the average electron count when the imaged region is completely dry, and I is the average electron count for the same region during wetting. The median precursor film thickness estimates for 230-, 285-, and 380-nm nanopillars are 14, 29, and 32 nm, respectively (Fig. 4). The smallest estimates for the precursor films were 8, 7, and 5 nm, respectively (Fig. 4). The ultrathin (<10 nm) precursor water films form from the condensation of water molecules evaporating from bulk liquid front (46) and the surface diffusion of water molecules from the edge of the droplet (47).
Fig. 4.
Thickness of the water film during wetting. (A) Schematics showing the precursor film and final water film after full wetting of nanopillars. Hprecursor and Hfinal are the thicknesses of the precursor and final water films, respectively. (B) Box and whisker plots of the precursor (black) and final water film (red) thicknesses for 230-, 285-, and 380-nm–tall nanopillars. The dashed blue line is the median, and the box’s bottom and top correspond to the 25th and 75th percentile values of the estimated thicknesses, respectively. Whiskers correspond to the lowest and the highest estimates of the water film thicknesses.
The reason for the median estimate of the precursor film thickness being roughly three times greater than the lower bound has to do with the speed at which the wetting dynamics are imaged. The nanopillars in the FOV become fully wet in a timescale ranging from less than a millisecond to a few hundred milliseconds, depending on how close the water droplet is to the FOV (SI Appendix, sections 7 and 8). Despite capturing image series at the highest possible rate of 300 fps, we are still limited by the camera speed when it comes to catching the true precursor water film during the wetting because the overall wetting process is quite fast. However, we emphasize that around 75% of the estimated precursor film thicknesses were less than 50 nm, suggesting that a precursor film forms before the nanopillars undergo wetting. At complete wetting, we expect the final water thicknesses to be the same as the nanopillar heights (SI Appendix, section 6). However, our estimates for the final thicknesses were approximately 50 to 80 nm less than their heights [i.e., approximately 180, 220, and 300 nm for 230-, 285-, and 380-nm–tall nanopillars, respectively (Fig. 4B)]. This slight deviation of the estimated water thicknesses from the expected thicknesses is because the single scattering approximation (Eq. 1) used to estimate the water thickness is less accurate for thick water films where electrons undergo multiple scattering events (44).

Conclusions

The spreading of water on a rough surface via wicking can be schematically described as shown in Fig. 5. First, an ultrathin precursor film forms on the hydrophilic surface (I). Second, the array of nanopillars acts like an interconnected network of nanocapillaries that pull and fill up with water (II, IIIa, IV). If the nanopillars are soft (e.g., 285- and 380-nm–tall nanopillars), they bend and cluster under the surface tension of water, promoting further capillary pull within the nanopillar clusters, which results in the formation of nanodroplets trapped between these clusters (IIIb). Once the level of the water outside the clusters reaches the same level as the nanodroplets, nanopillars relax and move back to their upright positions (IV). It is important to note that while the final thickness of the water film is determined by the height of the nanopillars (V), the thickness of the precursor film is independent of the nanopillar height (Fig. 4).
Fig. 5.
Wetting pathway of a hydrophilic nanopillar array. (I) A water droplet sitting on an array of nanopillars spreads on a dry hydrophilic rough surface by first forming a precursor film. (II) The nanopillars act as capillaries and pull the water, increasing its overall thickness. (IIIa) Short (or stiff) nanopillars wet uniformly as precursor film thickens, while (IIIb) taller (or softer) nanopillars bend and cluster due to capillary forces exerted by the water nanodroplets induced by capillary action. (IV) As the overall thickness of water film continues to increase, the nanopillars restore back to the original upright position, and (V) finally, the patterned surface becomes fully wet. Note that low contrast levels in the TEM images shown in Figs. 2 and 3 do not allow to resolve the menisci in thin water films. However, such menisci are expected between the nanopillars due to the capillary action, as noted in this schematic of the wetting pathway.
In summary, our observations provide key insights into nanoscale wetting dynamics of patterned surfaces, which is relevant to a broad range of natural and industrial processes. For example, the effects of the observed wetting of nanopillars are critical to nanofabrication processes used in the semiconductor industry (48). Densely packed three-dimensional nanostructures are at the core of future nanoelectronic components (49), whose fabrication requires multiple solution-based processing (wetting and drying) steps during which the nanostructures experience strong capillary forces that can damage them (50). Importantly, our results show that it is not the wetting that damages fragile nanostructures, but rather the drying stage of any solution-based process (SI Appendix, section 11) (40). More broadly, our approach to tracking how liquids interact with nanostructures provides a platform critical for revealing nanoscale details of wetting and can accelerate the design of tailored structured surfaces for a wide range of applications.

Materials and Methods

Liquid Cell Fabrication.

Liquid cells used in our experiments were assembled from two parts: a top and a bottom chip, whose fabrication steps are reported in our earlier studies (3942). Briefly, the top chips were fabricated from 300-µm–thick Si wafers with 25-nm–thick SiNx film on both sides, and the viewing windows were patterned using standard optical lithography and etching in a potassium hydroxide (KOH) solution. Nanopillars were patterned on one side of a 775-µm–thick Si wafer with 40-nm–thick SiNx film, from which the bottom chips were fabricated. During the postfabrication cleaning and drying of bottom chips, the nanopillars can inadvertently come in contact with water and collapse. To prevent this damage, we functionalized the nanopillars with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (catalog number AB111155, abcr GmbH, Karlsruhe, Germany) using a 30-min vapor-phase deposition. The chamber pressure and temperature at which this deposition was carried out were 300 to 400 Pa and 110 °C, respectively. This perfluorodecyltrichlorosilane functionalization renders the nanopillars hydrophobic [contact angle with a flat silicon dioxide substrate approximately 110° (51)], preventing any wetting during their fabrication.

TEM Imaging.

Before the wetting experiments, the bottom chips with nanopillars were treated with an air-plasma for 15 min to make them hydrophilic (SI Appendix, section 2). Next, the top and the bottom chips were aligned in a custom-built flow holder (SI Appendix, Fig. S1) and inserted into the TEM for imaging. All in situ liquid-phase TEM studies were done using a JEOL2010 200kV TEM, and movies were captured at 200 to 300 fps using Gatan OneView camera (Gatan, Inc.). The electron flux used for imaging was kept below 10 e ⋅ Å−2 ⋅ s−1 to minimize the beam-induced artifacts. For all wetting studies, the flow rate of deionized water (catalog number 320072, Sigma-Aldrich Co.) in the holder was maintained between 5 and 20 µL ⋅ min−1 using a syringe pump.

Image Processing.

Diameter distribution.

We used image processing codes written in Python 3.6 (52), using the NumPy (53), SciPy (54), Cython (55), OpenCV (56), matplotlib (57), scikit-image (58), h5py (59), and mahotas (60) libraries. To estimate the diameter distribution of the nanopillars, the original (4K × 4K resolution) images were inverted and blurred using a Gaussian filter (σ = 4 pixels). Each nanopillar is located using the Laplacian of Gaussian operator (61), and accurate segmentation of the nanopillars was done by applying Otsu’s thresholding (62) in a small region centered around each nanopillar. The distribution of nanopillar diameters is shown in SI Appendix, Fig. S3.

Drift correction.

The in situ TEM movies were drift corrected using custom codes written in Python 3.6 (52). First, the centers of all the nanopillars in all the images were located using the Laplacian of Gaussian operator (61). Second, all the nanopillars were labeled across time. Third, assuming that the location of each nanopillar should be the same for all the frames, the drift was calculated using the average change in position of all the nanopillars. Based on the calculated drift, the original image frames were translated appropriately for each movie. For further details on image processing, see SI Appendix, section 4.

Data Availability

All study data are included in the manuscript and/or supporting information.

Acknowledgments

This work was supported by the Singapore National Research Foundation’s Competitive Research Program funding (Grant NRF-CRP16-2015-05).

Supporting Information

Appendix (PDF)
Movie S1.
Movie showing the wetting of the 230-nm-tall nanopillars described in Figure 2A and Figure S5A. Imaging was done at 300 frames per second (fps), and every three consecutive frames were averaged to improve the image contrast.
Movie S2.
Movie showing the wetting of the 285-nm-tall nanopillars described in Figure 2B and Figure S5B. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.
Movie S3.
Movie showing the wetting of the 380-nm-tall nanopillars described in Figure 2C and Figure S5C. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.
Movie S4.
High-resolution movie showing the wetting of the 230-nm-tall nanopillars described in Figure 3A. Imaging was done at 200 fps, and every two consecutive frames were averaged to improve the contrast.
Movie S5.
High-resolution movie showing the wetting of the 285-nm-tall nanopillars described in Figure 3B. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.
Movie S6.
High-resolution movie showing the wetting of the 380-nm-tall nanopillars described in Figure 3C. Imaging was done at 300 fps, and every three consecutive frames were averaged to improve the contrast.

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Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 118 | No. 38
September 21, 2021
PubMed: 34535552

Classifications

Data Availability

All study data are included in the manuscript and/or supporting information.

Submission history

Accepted: August 4, 2021
Published online: September 17, 2021
Published in issue: September 21, 2021

Keywords

  1. liquid-phase transmission electron microscopy
  2. wetting
  3. capillary force
  4. wicking
  5. nanostructures

Acknowledgments

This work was supported by the Singapore National Research Foundation’s Competitive Research Program funding (Grant NRF-CRP16-2015-05).

Notes

This article is a PNAS Direct Submission.

Authors

Affiliations

Department of Physics, National University of Singapore, Singapore 117551;
Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 117557;
Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546;
Department of Physics, National University of Singapore, Singapore 117551;
Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 117557;
Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 117557;
Institute for Materials Research and Engineering, Agency for Science, Technology and Research, Singapore 138634;
Siddardha Koneti
Department of Physics, National University of Singapore, Singapore 117551;
Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 117557;
Interuniversity Microelectronics Centre (imec), Leuven B-3001, Belgium;
Frank Holsteyns
Interuniversity Microelectronics Centre (imec), Leuven B-3001, Belgium;
Department of Physics, National University of Singapore, Singapore 117551;
Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 117557;
Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546;
Department of Materials Science and Engineering, National University of Singapore, Singapore 117575

Notes

1
To whom correspondence may be addressed. Email: [email protected].
Author contributions: U.A., X.X., F.H., and U.M. designed research; U.A., T.G., and S.K. performed research; U.A., Z.A., X.X., and F.H. fabricated devices; U.A. and U.M. analyzed data; and U.A. and U.M. wrote the paper.

Competing Interests

The authors declare no competing interest.

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