IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 20 (2009) 305707 (9pp) doi:10.1088/0957-4484/20/30/305707 The hydrophobicity of a lotus leaf: a nanomechanical and computational approach Kantesh Balani 1,2,3 , Ruben Galiano Batista 2 , Debrupa Lahiri 2 and Arvind Agarwal 2 1 Materials and Metallurgical Engineering, Faculty Building, Room 409, Indian Institute of Technology Kanpur, Kanpur-208 016, India 2 Mechanical and Materials Engineering, Florida International University, EC 3464, 10555 W Flagler Street, Miami, FL-33174, USA E-mail: kbalani@iitk.ac.in Received 25 February 2009, in final form 20 May 2009 Published 8 July 2009 Online at stacks.iop.org/Nano/20/305707 Abstract The multi-scale microstructure of a lotus leaf is rendered non-wetting by micro-protrusions and nano-hairs present on its surface. The mechanical properties of the surface become important since the water droplet has to be supported on the micro-protrusions without wetting the surface. Current work correlates the non-wetting behavior of the lotus leaf with its mechanical properties (Young’s modulus and critical flexing stress) and areal spread of micro-protrusions on the leaf surface. Quasistatic nanoindentation of nano-hairs on the lotus leaf surface has shown a variation of elastic modulus between 359 and 870 MPa, which in turn dictates the critical flexing strength and consequent non-wetting. Computational fluid dynamics modeling is utilized to correlate wetting phenomena with the areal spread of micro-protrusions. A qualitative model is proposed for the way nature has chosen to render the lotus leaf surface non-wetting. (Some figures in this article are in colour only in the electronic version) 1. Introduction Nature is simple, but its science is not. The lotus leaf is known to grow at the bottom of ponds, but emerges above the water surface as if untouched by the contamination of the dirty water that it grows in [1]. The non-wetting lotus leaf is often related to extreme purity, which restricts bacteria and pathogens growing on its surface [2]. Water droplets roll over the leaf’s surface taking away all the dirt and leaving a clean surface behind. A high contact angle (>160 ◦ ) of the water droplet is attributed to two levels of microstructure of the lotus leaf [1, 3–6]. It is well documented in the literature that a material with the lowest surface energy renders a contact angle of 120 ◦ [5]. It is reasoned that only an increased surface roughness (apart from surface chemistry) can further enhance the contact angle of water with the surface [5]. 3 Author to whom any correspondence should be addressed. A variety of surfaces have been patterned for mimicking lotus leaf structure and rendering hydrophobic surfaces with high contact angles for potential applications such as water- repellent glass in the automobile industry, surgical anti- biofouling instruments, self-cleaning clothes and paints, molecular motors, and low-drag marine vessels [7–9]. Polycarbonate was roughened by a hydrophobic coating of POSS (polyhedral oligomeric silsesquioxane) resulting in a contact angle of greater than 165 ◦ for the structure [3]. PET fabric modified with silver/polystyrene (PS) displayed a superhydrophobic contact angle of 157 ◦ because of the enhanced roughness effect of hydrophobic PS [10]. Similarly, carbon nanotubes were deposited on cotton fabrics using ultrasonic irradiation to roughened the surface and achieve a superhydrophobic surface (>150 ◦ contact angle) [11]. Further, nanocasting has been performed on polyvinyl chloride (PVC) in two steps: (i) achieving a negative surface of a lotus leaf using polydimethylsiloxane (PDMS), and (ii) casting 0957-4484/09/305707+09$30.00 © 2009 IOP Publishing Ltd Printed in the UK 1