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LAMMP Seminar Video
Optical Tweezers Based Microrheology
Arthur Chiou, PhD (Applied Physics, California Institute of Technology)

Most of the materials, in general, are neither purely elastic nor purely viscous; they exhibits both elastic property (revealing its capability to store mechanical energy) and viscous property (manifesting its characteristic to dissipate energy, often in the form of heat). The study of the complex viscoelastic property of matter is known as rheology. Macro-rheology, or synonymously, classical rheometry, for the measurement of the bulk-average viscoelastic property of a material via a conventional rheometer often expresses the results in terms of either the magnitude of the complex viscosity|*|or both the real part (’) which is related to the elastic property and the imaginary part (”) which is related to the viscous property, both in the units of Pascal sec. (or poise; 1 poise = 0.1 Pa sec.), as a function of the shear rate. Likewise, micro-rheology allows us to probe the localized rheological properties of soft matter (often liquid) with one or more micron-size particles with a spatial resolution on the order of a few microns; the results are often expressed in terms of the complex modulus G* = G’ + i G” (where the real part G’ is the elastic modulus, and the imaginary part G” is the loss modulus), all in the unit of Pascal, as a function of angular frequency (). Micro-rheology can be broadly classified into the passive approach and the active approach. In the passive approach, known as particle tracking microrheology (PTM), the Brownian motion of one or more micron-size particles embedded in the sample medium are tracked and analyzed to deduce the viscoelastic property of the sample. In the active approach, the particle embedded in the sample medium is actively manipulated by an external force, and the dynamic response of the particle to the external force is measured to obtain the viscoelastic property of the sample solution. The shear rate (in Macrorheology) and the angular frequency (in microrheology) play a similar role in probing the time response of the materials, or the property of the material at different time scale. For example, many liquid behaves more elastic than viscous (i.e., G’ > G”) at relatively slow time scale, and vice versa (i.e., G” > G’) at relatively fast time scale. In this talk, I will give a very brief introduction and focus mainly on the active approach based on optical tweezers to trap and oscillate a micron-size particle embedded in the sample solution, where the amplitude and the relative phase of the oscillating particle are measured, and the viscoelastic properties of the sample solution (G’ and G” as a function of the oscillation frequency) are deduced. The methodology and the experimental results for different solutions, both from the literatures and from my own lab will be presented and a wide range of potential applications will be highlighted.

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