Active particle tracking microrheology using artificial thermal noise

Abstract

Passive particle-tracking microrheology (PTM) uses inherent Brownian motion of colloidal probe particles to characterize the mechanical properties of materials at micrometer and submicrometer length scales. In higher modulus materials (G* > 101 Pa), the particle experiences restricted Brownian motion such that its displacements during reasonable observation time scales drop below the spatial resolution of a typical optical microscope (∼10 nm). Thus, the passive PTM technique is generally limited to low modulus materials (G* ∼ 100 Pa). To overcome this, we have developed a form of active microrheology using electromagnetic tweezers that induce an artificial thermal noise on a superparamagnetic particle in the form of a random white noise signal. This signal imparts stochastic forces that drive resolvable displacements, which are greater than what is observed from thermal energy (kT) alone. The main advantage of this technique over traditional active microrheological methods is that the induced random motion of the particle allows one to use hydrodynamic models to obtain material functions without needing to measure a defined strain field. We implement the artificial thermal noise approach with a 35.1 Pa s Newtonian fluid and measure viscosities that are an order of magnitude higher than the typical passive PTM limit (100 Pa s).