Transport in Interfacial Systems: Biosensor, Polymer, Colloid & Bacteria
Life forms as we know emerged due to unique mechanical and transport properties exhibited by Soft Materials at interface. For instance, the interior of a cell is separated from outside environment by lipid bilayer which self-assembles in aqueous environment to create an interface by virtue of hydrophobic and hydrophilic ends. Examples of interfacial system includes polyelectrolyte multilayers, gel, colloid, surfactant, liquid crystal, variety of biological materials, bacterial systems etc. In this research work, we set out to understand transport in bioinspired interfacial system which are crucial for a variety of applications. Specifically, I have explored: Chapter 2) The role of analyte transport and reaction kinetics in dictating sensitivity and specificity of surface-based Biosensors (Fig. 1(a)) – Early identification of disease requires development of highly sensitive and specific detectors. Bodily fluids such as blood which is a common source of biomarkers has numerous entities which puts stringent requirements on sensitivity and specificity of biosensors. We numerically studied surface based microfluidic sensing platform to analyse the reaction and flow parameters most suited for a sensitive and specific detection. Chapter 3) Limitations of ‘Fluorescence Recovery After Photobleaching’ (FRAP) technique in estimating transport properties of ultrathin polyelectrolyte multilayer films (Fig. 1(b)) – FRAP is one of the most widely used technique to estimate transport parameters in biological cells and polymer films. A region of interest is irreversibly photobleached using high incident optical power and, thereafter, fluorescent recovery is initiated by fluorophores diffusing from the surrounding ‘fluorophore rich’ regions. The measurement of fluorescence recovery as a function of time enables the estimation of diffusion coefficient. An important conclusion from our numerical and experimental study is that, for ultrathin films of 100 nm thickness, classical FRAP analysis is not sufficient to probe diffusion. Chapter 4) Fluid flow and van der Waals force mediated pinning of polystyrene beads over micro-structured substrate (Fig. 1(c)) – We studied flow profile of polystyrene microsphere (5−10 μm) over crest shaped microstructures (height=1 μm) on glass ii substrate. A symmetry breaking in the flow profile of microsphere occurs even at low Reynolds number (1 – 5) which leads to the pinning of microspheres in downslope region. Further, a rapid flocculation of binary sized flowing mixture of microspheres is observed at relatively higher flow rates. The morphology of these microstructures are similar to the shape of plaque build-up in arteries, hence, these experiments suggest an important role of arterial morphology and size distribution of cellular debris in plaque formation. Chapter 5) A non-chemical technique to attach & transport cargo on bacteria (Fig. 1(d)) – Bacteria are the smallest (1 μm) living machines which are able to forage, communicate, and respond actively to the external stimuli. Researchers have shown cargo delivery by bacteria using chemical means, where, chemically coated microspheres stick to bacteria due to electrostatic interaction. Use of chemicals has compatibility issues and is not universal. We have developed a purely physical approach to attach oil droplet as cargo via sonication of entrapped bacteria on oil-water interface. These, cargo carrying bacteria can manoeuvre through obstacles and are able to transport oil droplets of size as large as 10 μm. To summarize, the focus of this thesis work was to understand transport at interfaces in different systems. We did numerical study of transport in surface based biosensors, where the analyte molecules in the solution medium is captured by receptor molecules coated on surface. Transport in such systems occur at liquid solid interface and is fundamentally limited by diffusive transport. To better understand diffusive transport and its implication we experimentally and numerically studied transport in water and polymer film interface using FRAP technique. We then explored convective transport of polystyrene tracer particles in microfluidic channel having micro-structured bottom. Our experiments revealed microspheres breaking off from the flow laminae even at low Reynolds number. Finally, we studied active mode of transport using bacteria, where, we discussed the limitation and possibility of cargo transport by bacteria.