Nandini Bhandaru

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

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.

Ordered alternating binary polymer nanodroplet array by sequential spin dewetting.

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.

Thermally Tailored Gradient Topography Surface on Elastomeric Thin Films

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.

Solvent-Vapor-Assisted Dewetting of Prepatterned Thin Polymer Films: Control of Morphology, Order, and Pattern Miniaturization

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.

Programmable Nanopatterns by Controlled Debonding of Soft Elastic Films

We report a facile patterning technique capable of creating nanostructures with different feature heights (hS), periodicities (λS), aspect ratios (AR), and duty ratios (DR), using a single grating stamp with fixed feature height hP and periodicity λP. The proposed method relies on controlling the extent of debonding and morphology of the contact instability features, when a rigid patterned stamp is gradually debonded from a soft elastic film to which it was in initial conformal contact. Depending on whether the instability wavelength (λF scales with the film thickness hF as λF ≈ 3hF) and the periodicity of the stamp feature (λP) are commensurate or not, it is possible to obtain features along each stamp protrusion when λF ≈ λP or patterns that span several stripes of the stamp when λF > λP. In both cases, the patterns fabricated during debonding are taller than the original stamp features (hS > hP). We show that hS can be modulated by controlling the extent of debonding as well as the shear modulus of the film (μ). Additionally, when λF > λP, progressive debonding leads to the gradual peeling of replicated features, which, in turn, allows possible tuning of the duty ratio (DR) of the patterns. Finally we show that by the simultaneous modulation of AR, DR, and hS, it becomes possible to create surfaces with controlled wettability.

Nano-patterned honey incorporated silk fibroin membranes for improving cellular compatibility

The desirable properties of a matrix for tissue engineering applications appears to shift from a minimal model of supporting cell growth to a more inclusive role as a matrix to trigger the regeneration process. This requirement has propelled efforts towards bio-functionalization of matrices both by chemical and textural modification of most advanced materials, including silk. Amongst different biomaterials, silk fibroin (SF) has been extensively used for various tissue engineering applications, because of its low immunogenicity and good mechanical strength, although low biological activity and a poor biodegradability rate has remained its major limitation for use in regenerative medicine. In the present study, the physical as well as biological properties of SF have been improved by incorporation of honey, a natural healing agent. Honey-blended SF films (HSF) demonstrated increased mechanical strength together with enhanced wettability, swelling ability and degradability without hampering the original mechanical and biological properties of both honey and SF. In addition, parallel-aligned nanostructures were produced over native SF and HSF using soft lithography to form two-dimensional nano-patterned membranes. HSF films demonstrated enhanced surface roughness in flat membranes and even more so in nano-patterned membranes when compared with native SF membranes which are essential for cellular adhesion and proliferation. Furthermore, biocompatibility and cellular behavior such as adhesion, proliferation, spreading and interaction of 3T3 fibroblast cells were improved using HSF when compared with using native SF membranes. Thus, HSF membranes with enhanced bio-physical properties are a potential material for use in regenerative medicine and tissue engineering applications.

Transition from Spin Dewetting to continuous film in spin coating of Liquid Crystal 5CB

Spin dewetting refers to spontaneous rupture of the dispensed solution layer during spin coating, resulting in isolated but periodic, regular sized domains of the solute and is pre-dominant when the solute concentration (Cn) is very low. In this article we report how the morphology of liquid crystal (LC) 5CB thin films coated on flat and patterned PMMA substrate transform from spin dewetted droplets to continuous films with increase in Cn. We further show that within the spin dewetted regime, with gradual increase in the solute concentration, periodicity of the isotropic droplets (λD) as well as their mean diameter (dD), gradually decreases, till the film becomes continuous at a critical concentration (Cn*). Interestingly, the trend that λD reduces with increase in Cn is exact opposite to what is observed in thermal/solvent vapor induced dewetting of a thin film. The spin dewetted droplets exhibit transient Radial texture, in contrast to Schlieren texture observed in elongated threads and continuous films of 5CB, which remains in the Nematic phase at room temperature. Finally we show that by casting the film on a grating patterned substrate it becomes possible to align the spin dewetted droplets along the contours substrate patterns.

Transfer printing of colloidal crystals based on UV mediated degradation of a polymer thin film

A versatile colloidal transfer printing method that can be used to transfer any type of particles (polymeric/inorganic) to any type of surface (hydrophobic/hydrophilic; smooth/rough) is reported. The method relies on UV mediated degradation of a sacrificial thin film of polymethylmethacrylate (PMMA), and does not necessitate the use of a transfer medium or modification of the target substrate.