Active particle tracking microrheology using artificial thermal noise

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* > 10(1) 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 (similar to 10 nm). Thus, the passive PTM technique is generally limited to low modulus materials (G* similar to 10(0) 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 (10(0) Pa s). (c) 2021 The Society of Rheology.

Automated summary

Passive particle-tracking microrheology is limited to low modulus materials. To overcome this limitation, we have developed an active microrheology method using electromagnetic tweezers to induce artificial thermal noise and drive measurable displacements.