Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2020 Jan 22;10(1):935.
doi: 10.1038/s41598-020-57410-2.

Wetting characteristics of Colocasia esculenta (Taro) leaf and a bioinspired surface thereof

Manish Kumar  1 , Rajneesh Bhardwaj  2
Affiliations

Wetting characteristics of Colocasia esculenta (Taro) leaf and a bioinspired surface thereof

Manish Kumar et al. Sci Rep. .

Abstract

We investigate wetting and water repellency characteristics of Colocasia esculenta (taro) leaf and an engineered surface, bioinspired by the morphology of the surface of the leaf. Scanning electron microscopic images of the leaf surface reveal a two-tier honeycomb-like microstructures, as compared to previously-reported two-tier micropillars on a Nelumbo nucifera (lotus) leaf. We measured static, advancing, and receding angle on the taro leaf and these values are around 10% lesser than those for the lotus leaf. Using standard photolithography techniques, we manufactured bioinspired surfaces with hexagonal cavities of different sizes. The ratio of inner to the outer radius of the circumscribed circle to the hexagon (b/a) was varied. We found that the measured static contact angle on the bioinspired surface varies with b/a and this variation is consistent with a free-energy based model for a droplet in Cassie-Baxter state. The static contact angle on the bioinspired surface is closer to that for the leaf for b/a ≈ 1. However, the contact angle hysteresis is much larger on these surfaces as compared to that on the leaf and the droplet sticks to the surfaces. We explain this behavior using a first-order model based on force balance on the contact line. Finally, the droplet impact dynamics was recorded on the leaf and different bioinspired surfaces. The droplets bounce on the leaf beyond a critical Weber number (We ~ 1.1), exhibiting remarkable water-repellency characteristics. However, the droplet sticks to the bioinspired surfaces in all cases of We. At larger We, we recorded droplet breakup on the surface with larger b/a and droplet assumes full or partial Wenzel state. The breakup is found to be a function of We and b/a and the measured angles in full Wenzel state are closer to the predictions of the free-energy based model. The sticky bioinspired surfaces are potentially useful in applications such as water-harvesting.

PubMed Disclaimer

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1
(a) A photograph of Colocasia esculenta (taro) plants in garden of our campus. Some leaves of an other plant (bottom left corner) can be seen as wet as compared to the leaves of the taro. (b) A plucked leaf of taro (c) Deionized water droplets on the leaf.
Figure 2
Figure 2
SEM image of the surface of the taro leaf. Zoomed-in view of the SEM image with systematic increase in magnification (a) 150X (b) 750X (c) 10000X (d) 30000X.
Figure 3
Figure 3
SEM images of the bioinspired surface taken from top and 25 ° tilt. The first and third row show the surfaces manufactured with different lengths of the side of the hexagonal cavity with 5 μ and 10 μ thickness of the wall of the cavity, respectively. The length of the side is shown on the top of each frame in first and third rows.
Figure 4
Figure 4
(a) Schematic of honeycomb geometry of the bioinspired surface, biomimicking first-tier structure found on the taro leaf (b) A unit cell of the honeycomb structure showing geometrical parameters a and b. AUTOCAD–R24.0 (www.autodesk.com) was used to create the CAD model.
Figure 5
Figure 5
Comparison between measured static contact angles at different values of b/a on the bioinspired surfaces and the predictions of a free-energy based model for a droplet in Cassie-Baxter state. The model was proposed by Patankar and here we employ it for the surfaces with hexagonal-cavities. b/a is the ratio of inner to the outer radius of the circumscribed circle to the hexagonal cavity.
Figure 6
Figure 6
Test of droplet sliding on the bioinspired surfaces with b/a = 0.65 (left) and b/a = 0.97 (right). Images for the tilt at 90° and 180° are shown in top and bottom row, respectively. The droplet does not slide and sticks to the surface even for the tilt at 180°.
Figure 7
Figure 7
(a) The contact line (shown as red) on the bioinspired surface with hexagonal cavities. The contact line can be approximated as a circle, as shown in the figure (black circle). (b) The contact line on a surface with micropillars. (c) Free body diagram of a pendant droplet with possible forces shown on the droplet. The droplet sticks to the bioinspired surface if the vertical component of the surface tension force acting on the contact line exceeds the droplet weight.
Figure 8
Figure 8
Static contact angle of the droplet (θ) on the bioinspired surface measured after it impacts on the surface with a given Weber number (We) and becomes sessile. The angle is plotted as a function of b/a i.e., the ratio of inner to the outer radius of the circumscribed circle to the hexagonal cavity. Different cases of We are considered and We ≈ 0 corresponds to a gently deposited droplet. Predictions of θ obtained by a free-energy based model are also plotted in which droplet is assumed to be in full Wenzel state. Left and right insets represent sessile droplet shapes for We ≈ 0, We = 4.5 and We = 15.9, marked by color coded arrows.
Figure 9
Figure 9
Variation of dimensionless wetted diameter of the droplet (d/d0) in the sessile state with respect to Weber number (We) for two cases of b/a = 0.65 and 0.98.
Figure 10
Figure 10
Impact dynamics of a microliter water droplet on the taro leaf at different Weber numbers (We) or impact velocities. The time instances of each row is indicated on the left. High-speed visualization movies are provided with the supplementary information. Supplementary information 9, 10, 11 and 12 correspond to cases of We = 1.1, 4.5, 9.1 and 15.9, respectively.
Figure 11
Figure 11
Impact dynamics of a microliter water droplet on bioinspired surfaces with different values of b/a, keeping Weber number as constant (We = 15.9, i.e., v = 0.82 m/s). High-speed visualization movies are provided with the supplementary information. Supplementary information 1, 2, 3 and 8 correspond to cases of b/a = 0.65, 0.78, 0.85 and 0.98, respectively.
Figure 12
Figure 12
Impact dynamics of a microliter water droplet on a bioinspired surface with b/a = 0.97, at different Weber numbers (We) or impact velocities. High-speed visualization movies are provided with the supplementary information. Supplementary information 4, 5, 6 and 7 correspond to cases of We = 1.1, 4.5, 9.1 and 15.9, respectively.
Figure 13
Figure 13
Regime map of the droplet fate on the bioinspired surfaces, namely, no-breakup and breakup, on Weber number (We) - b/a plane. A dashed line is shown as guide to the eye. The break-up is defined if the volume of the secondary droplet is more than 4% of the initial volume.
Figure 14
Figure 14
Time-varying instantaneous dimensionless droplet height (h/d0) on a bioinspired surface at different Weber numbers (We), (a) b/a = 0.65 (b) b/a = 0.98. A plateau signal of h/d0 implies that the droplet has become sessile.
Figure 15
Figure 15
(a) Experimental setup used in the present study. (b) A typical image obtained by side visualization of a water droplet resting on the surface of the taro leaf or the bioinspired surface. Droplet height (h) and wetted diameter (2R) are shown on the image.
See this image and copyright information in PMC

References

    1. Shirtcliffe NJ, McHale G, Atherton S, Newton MI. An introduction to superhydrophobicity. Adv. in Colloid and Interface Sci. 2010;161:124–138. doi: 10.1016/j.cis.2009.11.001. - DOI - PubMed
    1. Guo Z, Liu W, Su B-L. Superhydrophobic surfaces: From natural to biomimetic to functional. J. of Colloid and Interface Sci. 2011;353:335–355. doi: 10.1016/j.jcis.2010.08.047. - DOI - PubMed
    1. Liu X, Liang Y, Zhou F, Liu W. Extreme wettability and tunable adhesion: biomimicking beyond nature? Soft matter. 2012;8:2070–2086. doi: 10.1039/C1SM07003G. - DOI
    1. E J, et al. Wetting models and working mechanisms of typical surfaces existing in nature and their application on superhydrophobic surfaces: A review. Adv. Mater. Interfaces. 2018;5:1701052. doi: 10.1002/admi.201701052. - DOI
    1. Sarshar MA, Song D, Swarctz C, Lee J, Choi C-H. Anti-icing or deicing: Icephobicities of superhydrophobic surfaces with hierarchical structures. Langmuir. 2018;34:13821–13827. doi: 10.1021/acs.langmuir.8b02231. - DOI - PubMed

Publication types