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CURATOR
A pinboard by
Nandini Bhandaru

PostDoc Fellow at Indian Institute of Technology Kanpur studying superoleophobic surface application

PINBOARD SUMMARY

Nano fabrication by combination of top-down bottom-up techniques to achieve new surface properties

The key focus of my research is on creating and organizing things at the nanoscale. Nano represents 10-9 meters and to give an idea about how small it is, we are essentially talking about things roughly 1000th part of the width of human hair! Things at this length scale find wide application in various areas such as in all electronic devices, data storage media, optoelectronic devices, cancer therapeutics and so on. For most of the above applications rapid prototyping of nano structures over large areas (~cm2) is necessary, but the conventional machine based fabrication approaches fail as even most high end equipment have limitations. But on the other hand making things smaller actually adds new properties to a material; an insulating material may start conducting at nanoscale, a hydrophilic surface can become hydrophobic, gold which has yellow lustre becomes reddish or dirty brown when in the form of nanoparticles. Hence incorporating nano structures will have direct impact on a lot of application in our day to day life.

We have developed several inexpensive methodologies for fabricating ordered micro and nanostructures in polymeric materials which are so simple that they can be implemented in a high school level laboratory.

My current research is related to self-cleaning surfaces. An amazing example of such surface in nature is lotus leaf on which a water drop rolls off automatically taking along with it any dust on the surface thus providing self-cleaning ability. This is actually because lotus leaf has arrays of nanopillars which makes the surface super hydrophobic (water repelling) and superoleophobic (oil repelling). Many engineering structures starting from solar cell panel street lights to huge glass facades used in the construction of high rise buildings are often inaccessible for cleaning, especially in a country like India where dust and dirt can often tend to be oily in nature. Having nano structure coatings can enormously help in keeping the surfaces clean and even help maintain high efficiency in case of solar cells. Further, the presence on nano structure coating on top of a solar cell provides anti-reflection property (more light absorbing capability) which also enhances its power efficiency and increases the cell life by a couple of years. We are also trying to make oleophobic anti-fingerprint surfaces which will obviously find enormous application in all touch screen devices.

17 ITEMS PINNED

Directed ordering of phase separated domains and dewetting of thin polymer blend films on a topographically patterned substrate.

Abstract: Substrate pattern guided self-organization of ultrathin and confined polymeric films on a topographically patterned substrate is a useful approach for obtaining ordered meso and nano structures over large areas, particularly if the ordering is achieved during film preparation itself, eliminating any post-processing such as thermal or solvent vapor annealing. By casting a dilute solution of two immiscible polymers, polystyrene (PS) and polymethylmethacrylate (PMMA), from a common solvent (toluene) on a topographically patterned substrate with a grating geometry, we show the formation of self-organized meso patterns with various degrees of ordering. The morphology depends on both the concentration of the dispensed solution (Cn) and the blend composition (RB). Depending on the extent of dewetting during spin coating, the final morphologies can be classified into three distinct categories. At a very low Cn the solution dewets fully, resulting in isolated polymer droplets aligned along substrate grooves (Type 1). Type 2 structures comprising isolated threads with aligned phase separated domains along each substrate groove are observed at intermediate Cn. A continuous film (Type 3) is obtained above a critical concentration (Cn*) that depends on RB. While the extent of ordering of the domains gradually diminishes with an increase in film thickness for Type 3 patterns, the size of the domains remains much smaller than that on a flat substrate, resulting in significant downsizing of the features due to the lateral confinement imposed on the phase separation process by the topographic patterns. Finally, we show that some of these structures exhibit excellent broadband anti-reflection (AR) properties.

Pub.: 15 Jun '17, Pinned: 30 Aug '17

Robust omniphobic surfaces.

Abstract: Superhydrophobic surfaces display water contact angles greater than 150 degrees in conjunction with low contact angle hysteresis. Microscopic pockets of air trapped beneath the water droplets placed on these surfaces lead to a composite solid-liquid-air interface in thermodynamic equilibrium. Previous experimental and theoretical studies suggest that it may not be possible to form similar fully-equilibrated, composite interfaces with drops of liquids, such as alkanes or alcohols, that possess significantly lower surface tension than water (gamma(lv) = 72.1 mN/m). In this work we develop surfaces possessing re-entrant texture that can support strongly metastable composite solid-liquid-air interfaces, even with very low surface tension liquids such as pentane (gamma(lv) = 15.7 mN/m). Furthermore, we propose four design parameters that predict the measured contact angles for a liquid droplet on a textured surface, as well as the robustness of the composite interface, based on the properties of the solid surface and the contacting liquid. These design parameters allow us to produce two different families of re-entrant surfaces- randomly-deposited electrospun fiber mats and precisely fabricated microhoodoo surfaces-that can each support a robust composite interface with essentially any liquid. These omniphobic surfaces display contact angles greater than 150 degrees and low contact angle hysteresis with both polar and nonpolar liquids possessing a wide range of surface tensions.

Pub.: 13 Nov '08, Pinned: 30 Aug '17

Differential Behavior of Normal and Fibrotic Fibroblasts under the Synergistic Influence of Micropillar Topography and the Rigidity of Honey/Silk-Fibroin Substrates

Abstract: We report differential proliferation behavior of normal and fibrosis associated human oral fibroblasts on micropillar honey embedded silk fibroin substrates (HSF). Oral fibroblasts of different origins manifest differences in proliferation rate, morphology, and the cytoskeletal arrangement on HSF substrates with distinct topography (H, D, and S), stiffness, and honey concentration. It is observed that the proliferation rate is maximized for normal and inhibited for fibrosis associated fibroblasts on a HSF substrate surface with moderate height of ∼8.5 μm and 2% honey concentration. Molecular expression analysis reveals decrease in c-myc and p53 expression in later cells validating the inhibition of their proliferation rate, which is further correlated with the decreased Col I and Col III expression on this substrate. A substrate with enhanced interspacing and intermediate mechanical stiffness (0.57 ± 0.32 μN/nm) favors strong adhesion and stable cell–matrix interaction for normal cells, while exhibiting negative influence on fibrotic fibroblasts with poor adhesion and spreading capability. Decrease in vimentin, fibronectin expression, and cytoskeleton reorganization justify the poor stability of later cells on the optimized substrate, thereby allowing selective modulation of normal and fibrosis associated fibroblasts under the synergistic influence of honey concentration, topography, and rigidity of HSF substrates. The work highlights the possible therapeutic efficacy of honey based micropatterned substrates as smart patches for fast wound healing and in minimizing the chances of recurrence of precancer post oral tumor resection surgeries.

Pub.: 02 Aug '16, Pinned: 30 Aug '17

Rational Design of Hyperbranched Nanowire Systems for Tunable Superomniphobic Surfaces Enabled by Atomic Layer Deposition

Abstract: Superomniphobic surfaces display contact angles of θ* > 150° and low contact angle hysteresis with virtually all high and low surface tension liquids. The introduction of hierarchical scales of texture can increase the contact angles and decrease the contact angle hysteresis of superomniphobic surfaces by reducing the solid–liquid contact area. Thus far, it has not been possible to fabricate superomniphobic surfaces with three or more hierarchical scales of texture where the size, spacing, and angular orientation of features within each scale of texture can be independently varied and controlled. Here, we report a method for tunable control of geometry in hyperbranched ZnO nanowire (NW) structures, which in turn enables the rational design and fabrication of superomniphobic surfaces. Branched NWs with tunable density and orientation were grown via a sequential hydrothermal process, in which atomic layer deposition was used for NW seeding, disruption of epitaxy, and selective blocking of NW nucleation. This approach allows for the rational design and optimization of three-level hierarchical structures, in which the geometric parameters of each level of hierarchy can be individually controlled. We demonstrate the coupled relationships between geometry and contact angles for a variety of liquids, which is supported by mathematical models. The highest performing superomniphobic surface was designed with three levels of hierarchy and achieved the following advancing/receding contact angles with water 172°/170°, hexadecane 166°/156°, octane 162°/145°, and heptane 160°/130°.

Pub.: 30 Nov '16, Pinned: 30 Aug '17

Tunable hydrodynamic characteristics in microchannels with biomimetic superhydrophobic (lotus leaf replica) walls.

Abstract: 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.

Pub.: 22 Mar '14, Pinned: 30 Aug '17

Confinement induced ordering in dewetting of ultra-thin polymer bilayers on nanopatterned substrates

Abstract: We report the dewetting of a thin bilayer of polystyrene (PS) and poly(methylmethacrylate) (PMMA) on a topographically patterned nonwettable substrate comprising an array of pillars, arranged in a square lattice. With a gradual increase in the concentration of the PMMA solution (Cn-PMMA), the morphology of the bottom layer changes to: (1) an aligned array of spin dewetted droplets arranged along substrate grooves at very low Cn-PMMA; (2) an interconnected network of threads surrounding each pillar at intermediate Cn-PMMA; and (3) a continuous bottom layer at higher Cn-PMMA. On the other hand the morphology of the PS top layer depends largely on the nature of the pre-existing bottom layer, in addition to Cn-PS. An ordered array of PMMA core–PS shell droplets forms right after spin coating when both Cn-PMMA and Cn-PS are very low. Bilayers with all other initial configurations evolve during thermal annealing, resulting in a variety of ordered structures. Unique morphologies realized include laterally coexisting structures of the two polymers confined within the substrate grooves due to initial rupture of the bottom layer on the substrate followed by a squeezing flow of the top layer; an array of core–shell and single polymer droplets arranged in an alternating order etc., to highlight a few. Such structures cannot be fabricated by any stand-alone lithography technique. On the other hand, in some cases the partially dewetted bottom layer imparts stability to an intact top PS layer against dewetting. Apart from ordering, under certain specific conditions significant miniaturization and downsizing of dewetted feature periodicity and dimension as compared to dewetting of a single layer on a flat substrate is observed. With the help of a morphology phase diagram we show that ordering is achieved over a wide combination of Cn-PMMA and Cn-PS, though the morphology and dewetting pathway differs significantly with variation in the thickness of the individual layers.

Pub.: 23 Nov '15, Pinned: 30 Aug '17

Solvent-vapor-assisted dewetting of prepatterned thin polymer films: control of morphology, order, and pattern miniaturization.

Abstract: Ultrathin (<100 nm) unstable polymer films exposed to a solvent vapor dewet by the growth of surface instability, the wavelength (λ) of which depends on the film thickness (h(f)). While the dewetting of a flat polymer thin film results in random structures, we show that the dewetting of a prepatterned film results in myriad ordered mesoscale morphologies under specific conditions. Such a film undergoes rupture over the thinnest parts when the initial local thickness of these zones (h(rm)) is lower than a limiting thickness h(lim) ≈ 10 nm. Additionally, the width of the pattern grooves (l(s)) must be wider than λ(s) corresponding to a flat film having a thickness of h(rm) for pattern-directed dewetting to take place over surface-tension-induced flattening. We first present an experimentally obtained morphology phase diagram that captures the conditions where a transition from surface-tension-induced flattening to pattern-directed-rupture takes place. Subsequently, we show the versatility of this technique in achieving a variety of aligned mesopatterns starting from a prepatterned film with simple grating geometry. The morphology of the evolving patterns depends on several parameters such as the initial film thickness (h(f)), prepattern amplitude (h(st)), duration of solvent vapor exposure (SVE), and wettability of the stamp used for patterning. Periodic rupture of the film at regular intervals imposes directionality on the evolving patterns, resulting in isolated long threads/cylindrical ridges of polymers, which subsequently disintegrate into an aligned array of droplets due to Rayleigh-Plateau instability under specific conditions. Other patterns such as a double periodic array of droplets and an array of holes are also possible to obtain. The evolution can be interrupted at any intermediate stage by terminating the solvent vapor annealing, allowing the creation of pattern morphology on demand. The created patterns are significantly miniaturized in size as compared to features obtained from dewetting a flat film with the same hf.

Pub.: 19 Feb '15, Pinned: 30 Aug '17

Ordered alternating binary polymer nanodroplet array by sequential spin dewetting.

Abstract: We report a facile technique for fabricating an ordered array of nearly equal-sized mesoscale polymer droplets of two constituent polymers (polystyrene, PS and poly(methyl methacrylate), PMMA) arranged in an alternating manner on a topographically patterned substrate. The self-organized array of binary polymers is realized by sequential spin dewetting. First, a dilute solution of PMMA is spin-dewetted on a patterned substrate, resulting in an array of isolated PMMA droplets arranged along the substrate grooves due to self-organization during spin coating itself. The sample is then silanized with octadecyltrichlorosilane (OTS), and subsequently, a dilute solution of PS is spin-coated on to it, which also undergoes spin dewetting. The spin-dewetted PS drops having a size nearly equal to the pre-existing PMMA droplets position themselves between two adjacent PMMA drops under appropriate conditions, forming an alternating binary polymer droplet array. The alternating array formation takes place for a narrow range of solution concentration for both the polymers and depends on the geometry of the substrate. The size of the droplets depends on the extent of confinement, and droplets as small as 100 nm can be obtained by this method, on a suitable template. The findings open up the possibility of creating novel surfaces having ordered multimaterial domains with a potential multifunctional capability.

Pub.: 25 Nov '14, Pinned: 30 Aug '17

Thermally tailored gradient topography surface on elastomeric thin films.

Abstract: We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction varies across the length of the film (L), resulting in a surface with a gradient topography with progressively varying feature heights (hF). The steepness of the gradient can be controlled by varying the temperature gradient as well as the duration of precuring of the film prior to imprinting. The method has also been utilized for fabricating wettability gradient surfaces using a high aspect ratio biomimetic stamp. The use of a flexible stamp allows the technique to be extended for creating a gradient topography on nonplanar surfaces as well. We also show that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting.

Pub.: 05 Apr '14, Pinned: 30 Aug '17