Search

When a beam of light goes from a denser medium to a less dense medium, illuminates the interface of two mediums at an incident angle greater than the critical angle θ_C=sin^(-1)(n_2/n_1 ), light is totally internally reflected to the denser medium.At this moment, an evanescent field is formed, and it propagates along the surface of the denser medium, attenuating exponentially away from the surface. When a 3rd dense medium is brought into the vicinity, a portion of light passes into it, attenuating the reflected intensity. This phenomenon, called frustrated total internal reflection (FTIR), has been utilized in visualizing contact dynamics. It also provides a means to understand adhesion and friction between surfaces because they are known to depend sensitively on actual contact area as well as magnitude and spatial distribution of gaps of near contact down to the nanoscale. However, quantitative conversion of the reflected signal to gap thickness has only recently been achieved by proper analysis of the Fresnel equation with consideration of multiple reflections, transmissions, and polarization of light.This thesis presents a study of applications of frustrated total internal reflection (FTIR). First, we present a quantitative optical method to characterize dynamics of contact formation between two mediums based on theory of FTIR. The method is first validated by measuring height profile of convex lens in contact with flat prism surface and comparing with Hertzian theory. The method is then used to track the evolution of contact between a soft hemisphere brought into contact with a hard surface under water, as function of hemisphere stiffness, and surface wettability. We find an exponential rate of water evacuation from hydrophobic–hydrophobic (adhesive) surfaces that is 3 orders of magnitude smaller than that from hydrophobic–hydrophilic (non-adhesive) contact. This counterintuitive result comes from adhesive surfaces to more tightly sealing puddles of trapped water. Similar observation for a human thumb touching alternatively hydrophobic/hydrophilic glass surfaces, demonstrates generality of the mechanism and points to practical consequences. Second, we investigate the potential of FTIR to measure the thicknesses and deformations of thin Polydimethylsiloxane (PDMS) films. With tunable elastic modulus, deformations of thin films can be converted into pressure values, thus FTIR can be utilized as sensitive pressure sensor to measure bio-adhesion such as pressure distribution under gecko feet and mussel feet.

The adhesion strategies in biological systems have been widely studied and have been a great source of bio-inspirations for human adhesive technologies. Understanding adhesion systems depends not only on chemistry, but also on physics. The adhesive systems of geckos have many interesting properties, including controllability, self-cleaning, and ease of attachment and detachment. A large diversity of bio-inspired adhesives have been investigated in attempts to replicate these properties. For practical applications of these adhesives at macro scales, it is crucial to understand the distribution of stresses at interface of adhesive system. In this thesis, we present an optical sensor based on “frustrated total internal reflection(FTIR)” optics. In theory of FTIR, if an object is placed within less than several wavelengths distance from the interface between the first medium and the second medium, the evanescent wave will be different from the usual one, and it will pass energy across the second medium into object. According to this method, we can measure tiny deformation from soft materials such as PDMS, and calculate the tensile stress, thus this optical sensor is much more sensitive than the other biological sensors. Our sensor is composed of dove prism, 465nm blue LED light source, and several mirrors. The dove prism is used to achieve the FTIR at the interface between glass and PDMS thin film.We have made PDMS thin films by spin-coating, with thicknesses in the range of 50nm to 30µm. Upon calibration with amenable film, the sensor will be suitable for measuring the normal stress distributions under gecko feet or mussel feet