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Abstract
This thesis work addresses the microrheology of soft materials such as poly (ethylene) oxide solutions, type I collagen gel and endothelial cells using an oscillating optical tweezers technique. This advantageous technique is based on grabbing probe particles suspended in the media of interest with a tightly focused laser beam, measuring displacements and phase shifts of the particle's motion from which to calculate the viscoelastic properties of homogeneous and inhomogeneous mediums as the function of frequency and time. The technique gives the opportunity of taking local, one-particle measurements of a material's viscoelasticity, and non-local, dual-particle microrheology. Basic principles of the optical tweezers technique and our setup will be introduced in detail in Chapter 2.
Before introducing a true viscoelastic material in order to investigate its micromechanical properties under the application of an applied force, it is necessary to determine the elastic contribution introduced by the laser beam. For these purposes, one-particle experiments in a simple viscous fluid are reported and theoretically explained in Chapter 3.
The one-particle studies in purely viscous media are extended by introducing two-fluid systems that are composed of polymer [telechelic poly (ethylene) oxide and poly(ethylene) oxide] and water. Their experimentally obtained viscoelasticity will be supported by a theoretical Maxwell model, introduced in Chapter 4. The results will be compared with a well-developed Macrorheology technique that confirms results in the lower frequency range. The validation of the optical tweezers-based microrheology is demonstrated in Chapter 5; by comparison of a passive measurement based on thermal Brownian motion approach with the active, forced oscillation approach. These measurements require only tens of microliters of materials, making them useful in the study of biological materials.
In Chapter 6, the optical tweezers technique will be used for studying the local response of inhomogeneous materials with complex geometric structures. One such material of medical interest, collagen gels, was probed in order to characterize its viscoelastic and structural composition. The optical tweezers technique will be extended to use on endothelial cells to test the applicability of this probe as a non-invasive, effective rheometer.
