AFM Based Measurement Of Hydrodynamic Drag And Analysis Of Possible Artifacts

Micro/nanofluidics is one of the recently developing fields of science, engineering and technology. Because of the high surface to volume ratios, the flow on micro/nanoscale can be strongly affected by the interfacial properties and the associated phenomena. Brief knowledge of liquid drag, hydrodynamic boundary conditions as well as characterization of surfaces and interfaces are key factors to the design of effective and efficient micro/nano fluid transport systems. In this pursuit, numerous research efforts have been made and interesting phenomena, including, boundary slip, gaseous layers and nanobubbles etc have been reported. These phenomena have been identified to be affecting the hydrodynamic drag at the solid liquid interface.Atomic force microscopy has been proven to be an excellent state of the art technique for the study and characterization of surfaces and interfaces. It can be used for acquiring direct as well as indirect information of the phenomena occurring at solid liquid interface. This technique can give the information of the interface properties and the associated phenomena on precisely small scale i.e. sub nanometer. However, several studies have debated its use for the measurement of boundary slip. Factors, like virtual deflection, shape of the AFM force cantilever and stiffness of the cantilever, have been proposed to be affecting the interpretation of the boundary slip.The factors affecting the measurement of slip provide an excellent scope for further study. Similarly, the relation of the slip with wettability, the mutual disagreement of the studies on the magnitude of slip on smooth hydrophobic surfaces and deviation from the theoretical prediction of slip on such surfaces are the problems that worth further investigation. Moreover, the slip on rough surfaces with different wetting characteristics, use of magnetic field to control the drag at the solid liquid interface and characterize the spherical domains formed on the hydrophobic thin polymers films are of interest to the subject field.In this study, a detailed analysis of the factors affecting the measurement and interpretation of hydrodynamic drag at solid liquid interface was conducted with the help of atomic force microscopy(AFM). This study firstly reports the analysis of the slip on smooth hydrophilic surfaces. Different types of cantilevers, with varying shape and stiffness were used to study its effect on the measurement of boundary slip. Various shear rates were used in order to study the effect of shear rate on the boundary slip. It was found that the drag on the cantilever is an important factor which can affect the interpretation of the boundary slip. The results showed that the drag on the cantilever increases in a direct fashion with an increase in the drive velocity. Neglecting the drag on the cantilever can make a reasonable explanation to the shear rate dependent slip. Moreover, no evidence was found to support that the slip can be affected by shape of the cantilever or the applied shear rate.Afterward, the slip on smooth hydrophobic surfaces was also studied. Hydrophobic surfaces were prepared with greatest possible care. Slippage of water as well as electrolytic solutions on these surfaces was studied with the help of different shaped colloidal probes. The effect of shear rate upon the slip on smooth hydrophobic surfaces was also studied. The slip results showed that the no-slip condition is applicable to the smooth hydrophobic surfaces. The results of this study were precisely close to the theoretical studies. Furthermore, it was found that the no-slip condition on smooth hydrophobic surfaces deviates to apparent slip in the case of the presence of contaminates on the probe or the surface.Moreover, the hydrodynamic flow over rough hydrophilic and hydrophobic surfaces was also studied. The study revealed that the effect of surface roughness on slip is dominated over the effect of hydrophobicity. The magnitude of the slip on the rough hydrophilic and the hydrophobic was in minimal difference. The small difference in the values suggested that the main factor, which causes a slip like behavior on the rough hydrophobic surfaces is the roughness and not the hydrophobicity. Furthermore, the hydrodynamic flow analysis on heterogeneously rough surfaces suggested that the nanoasperities give rise to a local slip behavior. In the presence of the nanoasperities, the slip become highly local and varies according the topographic features of the surface.This study also reports the analysis of the effect of magnetic field(range B≤0.6T) on the slippage of water and electrolytes over hydrophilic and hydrophobic surfaces. The feasibility of magnetic treatment of liquid for controlling the boundary slip and hydrodynamic drag at solid liquid interface was analyzed with the help of AFM. The hydrodynamic and electrostatic force data was analyzed, carefully. However, the analysis did not produce any clear evidence to support that the magnetic treatment of water and electrolytic solutions,(B≤0.6T), can result persistent and detectable change in the hydrodynamic dynamic boundary conditions as well as the hydrodynamic viscous drag. The results further showed that the viscosity of water remains unaltered after the magnetic treatment. Furthermore, the electrostatic force measurements revealed that the surface charge did not change in considerable manner. This suggested that there was no significant change in the electrical conductivity and the hydrogen bonding.Additionally, the formation of spherical domains, on the hydrophobic polystyrene(PS) thin film spin casted on silicon substrate, was briefly characterized with the help of AFM. The results showed that the spherical domains on the PS thin film have some characteristics identical to that of micro/nanobubbles, including, sphericity, smaller contact angle(measured from the inside on the spherical domain), lower line tension, phase contrast as well as the coalescence phenomenon. However, the insensitivity to the contact force/lateral force, absence of the long range hydrophobic attraction force and the presence of contaminant like objects and scratches on these domains suggested that these domains are most likely blisters. Furthermore, the analysis of the PS film before and after the contact with water suggested that the film stretches, deforms and bulges after being exposed to water. A reasonable explanation to the nucleation of these spherical domains can be the permeation of the water into to the PS film-silicon interface. The penetration of water, into the PS film-silicon interface, through osmosis or defects present on the film causes detachment of the film from the substrate and thus the film stretches and bulges out, thus producing micrometric sized blisters.