Details

Autor(en) / Beteiligte

• Sun, Mengyue

Titel

Quantitative FTIR Imaging for Contact Dynamics Analysis

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2022

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• 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 ΘC=sin-1(n2/n1 ), 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.