Anvesh Gaddam

Surface-enhanced Raman spectroscopy (SERS) is a well-known label-free analytical technique for chemical and biological detection. Electromagnetic enhancement near the nanostructured surfaces contributes to the SERS performance. However, the substrate-based nanostructured SERS surfaces are conventionally fabricated using time- and resource-intensive techniques. A cost-effective and scalable process chain to fabricate disposable thermoplastic SERS substrates is investigated herewith. The surface design includes a multiscale topography that consists of single-tier laser-induced periodic surface structures (LIPSSs) encircled by two-tier hierarchical structures (HSs). Different types of LIPSS were investigated, generated by linear and circular polarisation of the laser. The process chain employed to produce high-performance SERS substrates includes, first, ultrashort laser-enabled fabrication of a textured metallic master and then replication of functional topographies on cyclic olefin copolymer (COC) using hot embossing and finally mask-coating the LIPSS ‘hot spot’ with gold for an electromagnetic enhancement. The HS topographies facilitated the superhydrophobic evaporation of samples to enrich the analytes onto the SERS ‘hot spot’ on the COC substrates. This multiscale design achieved a detection limit of up to 10⁻⁷ M for methylene blue and 4-MBA analytes. Highly regular linear LIPSS produced consistent Raman signals with a relative standard deviation (RSD) of 6.7%, ensuring reproducibility. In contrast quasi-regular triangular LIPSS exhibited enhanced signal intensity but with a relatively higher RSD of 10.2%. This scalable and cost-efficient approach has potential for widespread use in food and agricultural sectors, advancing the practical applications of SERS technology.

Orthopedic and dental implants made from Ti6Al4V are widely used due to their excellent mechanical properties and biocompatibility. However, the long-term performance of these implants can be compromised by bacterial infections. This study explores the development of hierarchically textured surfaces with enhanced bactericidal properties to address such challenges. Hierarchical surface structures were developed by combining microscale features produced by a microsecond laser and superimposed submicron features produced using a femtosecond laser. Microscale patterns were produced by the pulsed laser surface melting process, whereas submicrometer laser-induced periodic surface structures were created on top of them by femtosecond laser processing. Escherichia coli bacterial cells were cultured on the textured surface. After 24 h, a staining analysis was performed using SYTO9 and PI dyes to investigate the samples with a confocal microscope for live dead assays. Results showed bacterial colony formation onto the microscale surface textures with live bacterial cells, whereas the hierarchical surface textures display segregated and physically damaged bacterial cell attachments on surfaces. The hierarchical surface textures showed ∼98% dead bacterial cells due to the combined effect of its multiscale surface features and oxide formation during the laser processing steps. The efficacy of hierarchical surface textures in enhancing the antibacterial behavior of Ti6Al4V implants is evident from the conducted research. Such laser-based surface treatments can find potential applications in different industrial sectors.

Endoscopes are ubiquitous in minimally invasive or keyhole surgeries globally. However, frequent removal of endoscopes from the patient's body due to the lens contaminations results in undesirable consequences. Therefore, a cost-effective process chain to fabricate thermoplastic-based endoscope lenses with superior antifouling and optical properties is proposed in this research. Such multifunctional surface response was achieved by lubricant impregnation of nanostructures. Two types of topographies were produced by femtosecond laser processing of metallic molds, especially to produce single-tier laser-induced periodic surface structures (LIPSS) and two-tier multiscale structures (MS). Then, these two LIPSS and MS masters were used to replicate them onto two thermoplastic substrates, namely polycarbonate and cyclic olefin copolymer, by using hot embossing. Finally, the LIPSS and MS surfaces of the replicas were infiltrated by silicone oils to prepare lubricant-impregnated surfaces (LIS). Droplet sliding tests revealed that the durability of the as-prepared LIS improved with the increase of the lubricant viscosity. Moreover, the single-tier LIPSS replicas exhibited longer-lasting lubricant conservation properties than the MS ones. Also, LIPSS-LIS replicas demonstrated an excellent optical transparency, better than the MS-LIS ones, and almost match the performance of the reference polished ones. Furthermore, the LIPSS-LIS treatment led to superior antifouling characteristics, i.e., regarding fogging, blood adhesion, protein adsorption, and microalgae attachment, and thus demonstrated its high suitability for treating endoscopic lenses. Finally, a proof-of-concept LIPSS-LIS treatment of endoscope lenses was conducted that confirmed their superior multifunctional response.

Super-hydrophobic textured surfaces reduce hydrodynamic drag in pressure-driven laminar flows in micro-channels. However, despite the wide usage of non-Newtonian liquids in microfluidic devices, the flow behaviour of such liquids was rarely examined so far in the context of friction reduction in textured super-hydrophobic micro-channels. Thus, we have investigated the influence of topologically different rough surfaces on friction reduction of shear-thinning liquids in micro-channels. First, the friction factor ratio (a ratio of friction factor on a textured surface to a plain surface) on generic surface textures, such as posts, holes, longitudinal and transverse ribs, was estimated numerically over a range of Carreau number as a function of microchannel constriction ratio, gas fraction and power-law exponent. Resembling the flow behaviour of Newtonian liquids, the longitudinal ribs and posts have exhibited significantly less flow friction than the transverse ribs and holes while the friction factor ratios of all textures has exhibited non-monotonic variation with the Carreau number. While the minima of the friction factor ratio were noticed at a constant Carreau number irrespective of the microchannel constriction ratio, the minima have shifted to a higher Carreau number with an increase in the power-law index and gas fraction. Experiments were also conducted with aqueous Xanthan Gum liquids in micro-channels. The flow enhancement (the flow rate with super-hydrophobic textures with respect to a smooth surface) exhibited a non-monotonic behaviour and attenuated with an increase in power-law index tantamount to simulations. The results will serve as a guide to design frictionless micro-channels when employing non-Newtonian liquids.

Micro-dimples are a specific class of surface textures that are ubiquitous in a wide range of applications where functionalities such as wetting, hydrodynamic drag reduction, and sliding friction reduction are needed. However, fabrication of the micro-dimples through the existing manufacturing techniques involves the use of complex equipment and requires relatively long cycle times. Consequently, we report here proof-of-concept of a method that enables in fabricating micro-dimples in a facile and fast manner. The industrial sieve wire masks were employed to deform metallic sheets at discrete locations by the cold rolling process. The geometry of projections on the wire masks allowed the formation of ellipsoid shaped micro-dimples in two distinct patterns on metallic sheets. The finite element modelling of process-material interactions was accurately predicted the formation of micro-dimples under different reduction ratios. We also demonstrate that the micro-dimples can be easily replicated on polymer surfaces using textured metallic moulds, which facilitates low-cost mass production in microfluidic applications. The presence of micro-dimples on polymer surfaces has led to an increase in water contact angles and resulted in a substantial drag reduction in microchannels. From the tribological viewpoint, the obtained patterns and surface area fractions of the micro-dimples are attractive. These findings provide a promising way to create textured surfaces in an economical manner, with functionalities suitable for microfluidic and tribological devices.