Optical studies of micron-scale flows: holographic microscopy, optical trapping and superhydrophobicity

Microfluidics is a very recent branch of science and technology. The development and the success, it has had in the last 15 years, is mainly due to the concept of lab-on-a-chip. Those miniaturized devices, integrating one or more laboratory functions, have aroused great interest among several research areas as physics, chemistry, biology and bioengineering. When a fluid is confined in a micro or nano scale structure, its behaviour is strongly affected by its interactions with the surrounding surfaces. In this context, the theme of fluid/solid slippage has been widely studied both theoretically and experimentally. Innovative technologies to enhance the surface slippage by specifically designing the solid interfaces have reportedly demonstrated to be an effective way to reduce the fluid/solid friction. To this end, superhydrophobic surfaces have increasingly attracted the interest of the scientific and technological community thanks to the large wall-slippage they present for liquid water. Though their behaviour has been extensively investigated through several theoretical and numerical methods, the experimental approaches are still indispensable to test and understand the properties of these surfaces. However, the lack of a general predicting model is also due to the fact that no one of the several existing experimental techniques has shown up as a very reliable one. Indeed, the reported measurements of slippage still depends on the specific adopted method, thwarting attempts to corroborate the proposed theoretical and numerical schemes. Therefore, it is evident that a more sensitive and effective experimental technique is still missing. This thesis began and developed inside the wider project of setting up an innovative technique to investigate the fluid-solid slippage on superhydrophobic surfaces by means of optical tweezers. Even though this project is still going on, this work reports the steps performed along the long way towards this main goal and it consists of a collection of several researches involving different scientific fields as optics, microscopy, surface science, microhydrodynamics, microfluidics and microfabrication. The researches presented in this work can be separated in two main categories: i) holographic micromanipulation and microscopy, ii) superhydrophobicity. Accordingly, the thesis is divided in two Parts. In the first Part, holography is adopted to accomplish two completely different tasks. Projecting an hologram on a spatial light modulator, the light intensity of a coherent source is modified at will so that several optical traps are generated in the sample volume. In this scheme, the laser light is an input of the microscope system and it used as an active tool to manipulate micron-scale objects and investigate the surrounding fluid. On the other hand, illuminating the sample through a monochromatic plane wave generates an hologram, recorded on CMOS camera, that contains the information of the light intensity distribution in the entire sample volume. In this case, the coherent light is the output of the microscope system and provides a tool for three-dimensional visualization and analysis of the examined sample. In such a context, if we regard a trapped bead as a local probe of fluid properties, holographic optical trapping allows us to trap multiple particles at the same time and to freely move them throughout the sample volume. The second Part of the thesis deals with the experimental characterization of the superhydrophobicity and slippage properties of silicon surfaces. Micro particle image velocimetry is adopted to provide a reference standard characterization of microgrooved silicon superhydrophobic samples that would be very useful in view of a validation of holographic trapping measurements of fluid/solid slippage. However, what was initially conceived as a standard approach has revealed some interesting aspects that have been deeply investigated, as the role of the curvature and position of the liquid/air meniscus inside the microgrooves and the detrimental effect of the contamination of the liquid/air interface. As regards microfabrication, we also discuss an easy-to-implement and very cheap procedure to turn a silicon flat sample into a superhydrophobic surface via porosification. Demonstrating the capabilities of digital holographic microscopy and holographic tweezers to locally probe the fluid properties in terms of high accuracy measurements and high spatial resolution, the work presented in this thesis paves the way for superhydrophobicity characterization via holographic tweezers and digital holographic microscopy.