Development of a Single-particle Tracking Microrheometry Method by Incorporating Magnetic Tweezer to Total Internal Reflection Microscope

Rheology is a multidisciplinary subject connecting physics, chemistry, materials, and engineering. It plays an important role in many industrial processes such as paint, food technology, oil recovery, and processing of plastics. The viscoelastic properties of materials can be determined by a variety of types of rheometers. An old fashioned rheometer is based on how fast a solid ball with a known size and density falls through a medium. Nowadays, viscoelastic materials were mostly studied with mechanical rheometers in different geometries depending on the range of strain and moduli to be measured. Over the last ten years a new approach of using small probes to locally measure mechanical properties has been gradually developed. Owing to their small probe size (∼μm) and a tiny amount of material required, this new method is called Microrheology. Despite strong impetus in driving the minimization of rheometers, it is still rather difficult to accurately probe rheological behaviours of a material at the micro scale without alternating/disturbing its structure. This thesis describes the development of a new type of microrheolometer and its application in the study of mechanical properties of soft matters. The thesis is outlined as follows.;First, we introduce the principles of some pre-existing microrheology techniques and report our idea how to develop a new microrheometer by incorporating the magnetic tweezer into our recently established total internal reflection microscope (TIRM) so that we can accurately measure rheological properties of some complex fluids. Its principle is briefly described as follows. In a TIRM measurement, an evanescent wave is generated by the total internal reflection of an incident laser light at a solid (glass slide )/liquid interface. The intensity of such a wave exponentially decays with the distance away from the interface. When a paramagnetic bead (∼μm) is placed close to the interface within ∼100 nm, it scatters sufficient light for detection. As expected, the scattered light intensity exponentially decays as the bead moves away from the interface, making such a method extremely sensitive to the bead movement. By measuring the scattered intensity, TIRM can track the bead position relative to the interface. The incorporation of a magnetic tweezer enables us to effectively move the probe bead. The measurement of its displacements can quantitatively lead to viscoelastic moduli of a give material. In our design, two sets of four electromagnetic pole pieces are symmetrically arranged in the upper and lower planes of the sample cell to achieve a three-dimensional position control. Highly precise force can be achieved by a real-time control of the electric current. In such an arrangement, we are able to drive the embedded paramagnetic bead to move in different fashions and monitor its response in the medium by a combination of the magnetic tweeze and the evanescent wave-scattered particle tracking.;In the second part, we describe the details of our microrheology instrumentation, including the sample cell setup, the magnetic tweezer construction, the design of control circuit and the tracking programs. Such a self-developed instrument can exert a magnetic force on the paramagnetic bead either as an oscillatory wave with a frequency range 0.1–10 Hz or as a pulse wave for creep experiments. By measuring the magnetic force exerted on the probe bead in sucrose solution based on the classic viscous drag approach, we demonstrate that this instrument can generate an upwards magnetic force up to 4 pN.;Finally, we demonstrate how to apply such a single-particle tracking microrheology method to measure the micro-viscoelasticity of gelatin solutions during the sol-to-gel transition. We investigated the time and temperature dependence of the storage and loss modulus in the frequency range 0.5–50 rad/s in both the creep and oscillatory experiments, and compared our results with those obtained using bulk rheology. The mechanical properties of these two micro- and macro-approaches are similar. It should be stated that a combination of the magnetic tweezer and TIRM, i.e., the current microrheology technique, has a number of advantages, including a small moving distant (101–10 2 nm) and a small sample volume (microliters). More importantly, this novel method enables us to probe the fragile materials (e.g, weak physical gels) without damaging their micro-structures, where a conventional rheometer will significantly alternate their structures during the measurements. Therefore, this newly developed noninvasive microrheometer opens a door for direct and in-situ measurements of rheological behavior of soft matters including biological samples at the micro-scales, potentially leading to a better correlation among intermolecular forces, microstructure and mechanical behavior of soft materials.