Abstract—Superhydrophobic surfaces are abundant in nature. Several surfaces such as wings of butterfly, legs of water strider, feet of gecko and the lotus leaf show extreme water repellence behaviour. Self-cleaning, stain-free fabrics, spill-resistant protective wears, drag reduction in micro-fluidic devices etc. are few applications of superhydrophobic surfaces. In order to design robust superhydrophobic surface, it is important to understand the interaction of water with superhydrophobic surface textures. In this work, we report a simple coating method for creating large-scale flexible superhydrophobic paper surface. The surface consists of multiple layers of silanized zirconia microparticles decorated with zirconia nanoparticles. Water contact angle as high as 159±1 0 and contact angle hysteresis less than 8 0 was observed. Drop impact studies on superhydrophobic paper surface were carried out by impinging water droplet and capturing its dynamics through high speed imaging. During the drop impact, the Weber number was varied from 20 to 80 by altering the impact velocity of the drop and the parameters such as contact time, normalized spread diameter were obtained. In contrast to earlier literature reports, we observed contact time to be dependent on impact velocity on superhydrophobic surface. Total contact time was split into two components as spread time and recoil time. The recoil time was found to be dependent on the impact velocity while the spread time on the surface did not show much variation with the impact velocity. Further, normalized spreading parameter was found to increase with increase in impact velocity. Keywords—Contact angle, contact angle hysteresis, contact time, superhydrophobic. I. INTRODUCTION NDERSTANDING the dynamic behaviour of droplets impacting on solid surfaces is of great interest due to its various technological applications such as inkjet printing, spray painting, superhydrophobic coatings etc. [1]-[4]. In this context, high speed imaging has become an attractive tool to understand the drop impact at short time scales in the range of few milliseconds thereby enabling better design of such surfaces [5]. When drop impacts the solid surface, it was observed that drop undergoes various morphological transformation such as spreading, recoiling, and bouncing depending on the nature of substrate and the liquid [6]-[9]. The parameters such as contact time, maximum spread diameter, satellite drop formation, resonant frequency, oscillation dynamics etc. have become important parameters to fabricate surfaces with superior functional benefits. Richard et al. Abinash Tripathy is with the Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore, India (e-mail: atripathy8@gmail.com). developed an expression for the contact time (τ = (ρR 3 /γ) 1/2 ) on a non-wetting surface by performing the droplet impact experiments at different Weber numbers [9], [10]. They also showed the non-dependence of contact time on droplet impact velocity over a wide range of velocities. Okumura et al. could predict the maximal deformation and the contact time of an impacting droplet onto a superhydrophobic surface using scaling arguments [8]. In the same vein, Clanet et al. found scaling law (D max /D 0 =We 0.25 ) by balancing between gravity and the surface forces [7]. Another critical factor which affects the impinging behaviour is the flexible nature of the substrate. Most of the studies in the literature have been limited to rigid substrate [1]-[3], [5], [11]-[14]. Thus, in this work, we report a simple, cost-effective method to fabricate large-scale, micro-nano structured flexible superhydrophobic surface. Superhydrophobic properties were studied through drop impact studies using high speed imaging. The contact time and normalized spreading parameter were measured for water droplet on the superhydrophobic surface. To unravel the critical parameters, we split the contact time into spread time and recoil time. Also, the normalized spreading parameter at different Weber number has been obtained from the experimental data. II. FABRICATION METHOD In this work, flexible superhydrophobic surfaces are prepared using sol-gel technique. Two different sizes of Zirconia nanoparticles were used. First of all, 1 ml of 1H, 1H, 2H, 2H Octyltriethoxy silane was added to 40 ml of Ethanol. The solution was then stirred for 2 hours to ensure proper mixing. After that Zirconia microparticles (0.9 g, ~ 3 microns) was added to the solution and stirred for 30 minutes followed by the addition of Zirconia nanoparticles (0.4 g, ~ 180 nm). The solution was further stirred for one hour. Then the mixture was spread on paper using a syringe and the surface was allowed to dry in room temperature. Fig. 2 represents the representative FESEM image of hierarchical structures on the paper surface. After that, experiments were performed on the surface to characterize the surface. III. EXPERIMENTAL SET UP After the fabrication of the nano structured hierarchical surface, contact angle and contact angle hysteresis measurement were carried out using a Goniometer. A 10 µL drop of water was placed on the superhydrophobic Drop Impact Study on Flexible Superhydrophobic Surface Containing Micro-Nano Hierarchical Structures Abinash Tripathy, Girish Muralidharan, Amitava Pramanik, Prosenjit Sen U World Academy of Science, Engineering and Technology International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:10, No:7, 2016 772 International Scholarly and Scientific Research & Innovation 10(7) 2016 scholar.waset.org/1999.2/10004796 International Science Index, Materials and Metallurgical Engineering Vol:10, No:7, 2016 waset.org/Publication/10004796