STUDYOFMINIATURIZATIONOFSURFACEPATTERNSANDSELF-ORGANIZATIONOF CARBONACEOUSNANOPARTICLESON SOFT THIN FILMS AND THEIRAPPLICATIONS
Date : 21st September 2021
Venue : Online
Time : 4.00 PM
MS TEAMS LINK: https://teams.microsoft.com/l/meetup-join/19%3ameeting_ZTFhYzhjMGMtNDUwZi00NTEzLThiNjUtMzNkNTdiNjljNzZh%40thread.v2/0?context=%7b%22Tid%22%3a%22624d5c4b-45c5-4122-8cd0-44f0f84e945d%22%2c%22Oid%22%3a%222dba7fce-4ddb-4d24-ac09-5d4ebf4b3553%22%7d
Abstract:-
Thin films are of paramount interest in daily customary life. Patterns formed in thin films are of more advanced importance. These micro and nano patterns on solid and liquid film is not only used in microelectronics but has emerged as an winner in other recent developments such as in fabrication of lab-on-chip devices for biomedical purposes, coating and adhesives as functional and self-cleaning material, various scaffolds for tissue engineering, and many more. The budding application of these patterns in thin film have led to the replacement of hard, exuberant and costly forms of lithography techniques giving way to low cost “self-organization” and “self-assembly” technique. Self-organization requires external energy source and therefore is dissipative and non-equilibrium but self-assembly is a spontaneous equilibrium process. A thin film in a confined region due to competition between intermolecular forces and surface tension becomes unstable and patterns arise on the surface of thin liquid films due to the competitive nature between the excess intermolecular interactions and interfacial tension. Another kind of pattern formation is observed at thesurface of a soft elastic film when the film is brought in close proximity of a contactor. The interaction forces between the film and the contactor destabilizes the film surface, while the stored elastic energy tries to restore the film to original configuration. These antagonistic forces give rise to instabilities in the surface and thus results in meso/nano scaled morphological surface feature.Miniaturization of patterns is a new addition to this flock of studies. The process of making miniaturizationof these patterns formed due to self-assembly is of much interest due to its ability to create small size and portability, low sample size, volume and cost reductionThe formation of miniaturized surface morphologies over a soft thin elastic film of micron range thickness (h), when cast on various patterned substrates created from: naturally available intrinsically rough water lily leaves, commercially available sinusoidally patterned compact disks and cubic boxes created by electron beam lithography were investigated experimentally.The evolving surface patterns formed had varying structures of columns to labyrinth to cavities during the adhesion and debonding cycle,
irrespective of which the engendered dominant wavelength (λ) remained at a miniscule length scale and is depended solely on its RMS roughness.It was observed experimentally that the length scales of the patterns formed decreased linearly with increase in substrate amplitude and regardless of the substrate pattern geometry used depended solely on its RMS roughness such that λ⁄h=2.96-18.2*(RMS/h).The study indicates the redundancy in creating very costly regular geometrical patterned substrates for miniaturization and instead advocates the use of intrinsically rough natural surface replicasto be used as patterned substrates for cost-effective fabrication of miniaturized surface patterns at such soft interfaces.The spontaneous self-assembly of the carbon nano-particles-graphene and carbon nanotube intensify over the self-assembledpatternsformed on the thin polymeric film. The dewetting dynamics of the underlying filminfluences the formation of unique self-assembled graphene patterns at the interface between graphene and a thin polystyrene film by various approaches-graphene-polymer composite thin film, polymer thin film with graphene nanoparticles as a bilayer and graphene nanoparticles drop-cast over the array of dewetted holes on polymeric thin-film. When a minute amount of graphene dispersed in a solvent is added on PS and spin-coated into a thin film, the concentration gradient caused by centrifugal force leads to a Marangoni flow that in conjunction with the dewetting of the underlying PS at the edges lead to a series of very interesting, striking and self-assembled morphologies of graphene.At the particle enriched zone near the center, the graphene-particles exhibit morphologies ranging from folds, wrinkles, flakes, onion-rings to blob structures depending on the aspect ratio. The increase in graphene nanoparticle concentration and the annealing temperature in this regime leads to formation of clusters. The graphene-particles thrown to the periphery are found to march back towards the central portion leading to the unique formation of very ordered nano-scale scratches on the PS substrate. The nano-particles that end up into the confined rims in the intermediate region get twisted into nano-ribbons and dendrimers.The increase in concentration of graphene nanoparticles and temperature leads to formation of fingering morphologies in this region. The increase in the temperature near the dewetting regime caused the polymer in presence of high concentration of graphene nanoparticles to form a floral pattern on the surface of the film.The polystyrene-graphene nanoparticles composite when spin-coated together to form a thin film over a substrate, it was observed that these nanoparticles self-assemble in a Voronoi tessellation of ribbon-like structure along with the suppression of dewetting of thin polystyrene film at this stage. The graphene nanoparticles dispersed in solvent were casted over the instability based dewetting patterns formed on the surface of the thin polystyrene film causing the graphene nanoparticles to arrange in a
polygonal array of dots prompted by the dynamics and morphology of dewetting. The reversals of the dewetted patterns were observed in this process because of the solvent evaporation and Marangoni flow. Moreover, without the aid of any high-end instrumentation or process, different origami structures were formed by the presence of different forces and the underlying dewetting dynamics.The instability of a polymeric thin film, when cast either as a mobile underlying support-layer of a bilayer assembly, as a mixture, or as a template, affects the self-assembly and arrangement of carbon nanotube (CNT) nanoparticles dispersed over/with it. The interplay of the different forces, like centrifugal, dewetting, Marangoni, electrostatic, assist the mobile nanoparticles in arranging themselves into circular ring-like structures. The congruence of the length-scales of the self-assembled circular ring and the dewetted length-scales of the thin polystyrene film reveals that the influence of dewetting dynamics is majorly responsible for the incipience of ring formations.The nanoparticles not involved in circular-ring-self-assembly formation are driven by Marangoni force (effective due to difference in surface tension values because of nanoparticle concentration difference) and engender nano-scratches over the polymeric surface. The bending of the carbon nanotubes is an energetically favoured self-organization process controlled by the shape formation energy.Finally, surface corrugations such as columns, labyrinths and cavities evolved spontaneously due to instability on thesurfaces of a thin soft elastic PDMS film in adhesive contact with a contactor along with the oleophilic nature of the film help in selective adsorption of the oil from the oil-water mixture into the intervening surface spaces. The increase in RMS surfaceroughness brought about either by increase in adsorbent weight or by formation of a columnar structure (instead of cavities or labyrinths) was found to have a significant incremental effect on the true surface area creation by formation of increased number of oil adsorption sites: leading to higher oil penetration and thus, higher oil adsorption. For 0.3wt% of adsorbent, adsorption was found to be maximum for columnar structures at 23%, followed by cavities at 15% and minimum with labyrinth at around 12%. The ratio (dp/dd) of the substrate pore size dimension (dp) to the surface oil droplet dimension (dd) in crude oil, motor oil, diesel and petrol were found to be progressively lower and hence the maximum adsorption and penetration in the columnar patterns was recognized in case of crude oil. Additionally, the circular rings of CNTs formed throughout the PS surface, can be used to identify trace amounts of oil from an oil-water mixture by selective adsorption. This thesis contributes on creating spontaneous patterns on soft thin film embedded with/without carbonaceous nanoparticles via different approaches with enhanced surface properties