Tunable hydrodynamic characteristics in microchannels with biomimetic superhydrophobic (lotus leaf replica) walls† Ranabir Dey, a Kiran Raj M., b Nandini Bhandaru, c Rabibrata Mukherjee c and Suman Chakraborty * ab The present work comprehensively addresses the hydrodynamic characteristics through microchannels with lotus leaf replica (exhibiting low adhesion and superhydrophobic properties) walls. The lotus leaf replica is fabricated following an efficient, two-step, soft-molding process and is then integrated with rectangular microchannels. The inherent biomimetic, superhydrophobic surface–liquid interfacial hydrodynamics, and the consequential bulk flow characteristics, are critically analyzed by the micro- particle image velocimetry technique. It is observed that the lotus leaf replica mediated microscale hydrodynamics comprise of two distinct flow regimes even within the low Reynolds number paradigm, unlike the commonly perceived solely apparent slip-stick dominated flows over superhydrophobic surfaces. While the first flow regime is characterized by an apparent slip-stick flow culminating in an enhanced bulk throughput rate, the second flow regime exhibits a complete breakdown of the aforementioned laminar and uni-axial flow model, leading to a predominantly no-slip flow. Interestingly, the critical flow condition dictating the transition between the two hydrodynamic regimes is intrinsically dependent on the micro-confinement effect. In this regard, an energetically consistent theoretical model is also proposed to predict the alterations in the critical flow condition with varying microchannel configurations, by addressing the underlying biomimetic surface–liquid interfacial conditions. Hence, the present research endeavour provides a new design-guiding paradigm for developing multi-functional microfluidic devices involving biomimetic, superhydrophobic surfaces, by judicious exploitation of the tunable hydrodynamic characteristics in the two regimes. I Introduction The term ‘biomimetics’, as coined by Otto Schmitt in 1957, refers to the technological eld pertaining to the development of functional surfaces and devices, which mimic the behaviour of biological surfaces, and bio-physical processes, inherent in various plant and animal species. 1 The cornerstone of biomi- metics is the need for replicating the intricate interplay of the structural, physical and chemical properties, of various natural surfaces to mimic a myriad of naturally observable physical features like superhydrophobicity, controllable or reversible adhesion, drag reduction, structural colours, and aerodynamic properties. 2,3 It goes without saying that such biomimetic surfaces, endowed with the aforementioned attributes, are indispensible for a wide range of engineering applications like the development of self-cleaning coatings, development of anti- (bio)fouling surfaces, underwater drag reduction, energy conversion and conservation, transport of liquid microdroplets, development of smart adhesive tapes, development of antifog- ging coatings and antireection coatings, and development of optical devices for solar cells, only to name a few. 2–6 However, arguably the most multi-functional of all the biomimetic surfaces are the low adhesion, superhydrophobic surfaces, which exhibit extreme water repellence and self-cleaning properties, popularly categorized as the ‘Lotus effect’. 2,3 Natu- rally, lotus (Nelumbo nucifera) leaves achieve such striking superhydrophobicity due to the ubiquitous complex hierar- chical micro- and nano-structures, constituting of microscale convex structures (papillae) covered with nanoscale, three- dimensional wax tubules. 7,8 Over the years, various techniques were standardized for fabricating articial surfaces demon- strating the aforementioned ‘Lotus effect’, 9–12 but to date, the two-step, so-molding process, involving the fabrication of negative and subsequent positive replicas of natural lotus a Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, West Bengal, India. E-mail: suman@mech.iitkgp.ernet.in b Advanced Technology Development Centre, Indian Institute of Technology Kharagpur, Kharagpur 721 302, West Bengal, India c Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, West Bengal, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4sm00037d Cite this: Soft Matter, 2014, 10, 3451 Received 7th January 2014 Accepted 17th February 2014 DOI: 10.1039/c4sm00037d www.rsc.org/softmatter This journal is © The Royal Society of Chemistry 2014 Soft Matter, 2014, 10, 3451–3462 | 3451 Soft Matter PAPER