Paper Number
MF4 

Session
Micro/Nano Fluidics and Probe Rheology

Title
No tracking necessary: Probe microrheology by differential dynamic microscopy

Presentation Date and Time
February 14, 2017 (Tuesday) 4:50

Track / Room
Track 3 / White Ibis

Authors (Click on name to view author profile)

1. Bayles, Alexandra V. (University of California, Santa Barbara, Department of Chemical Engineering)
2. Squires, Todd M. (University of California, Santa Barbara, Department of Chemical Engineering)
3. Helgeson, Matthew E. (University of California, Santa Barbara, Department of Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Alexandra V. Bayles, Todd M. Squires, and Matthew E. Helgeson
Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106-5080

Speaker / Presenter 
Helgeson, Matthew E.

Text of Abstract
Multiple particle tracking (MPT) has become a ubiquitous tool for probe microrheology, in which the mean squared displacement (MSD) of Brownian particles can be used to extract viscoelastic information from complex fluids including colloids, polymers, gels and glasses. However, MPT is not without its drawbacks – it is limited to optically dilute suspensions, to probe particles with known intensity profiles, and by subjective user inputs that must be chosen in an ad hoc manner, resulting in significant information loss. In this work, we report an alternative method for extracting MSDs and linear viscoelastic material functions from passive probe video microscopy using differential dynamic microscopy (DDM). By correlating the intensity fluctuations of a video micrograph in Fourier space, DDM can be used to extract the self-intermediate scattering function (SISF) in systems that would otherwise be difficult to measure using traditional photocorrelation spectroscopy or probe microrheology. We use a simple theoretical framework to show that the SISF obtained by DDM on dilute suspensions can be inverted to obtain the real-space MSD over length and time scales comparable to MPT. As demonstrative examples, we apply DDM to passive microrheology videos of dilute probes in Newtonian fluids, viscoelastic wormlike micelles, and crosslinking polymer gels. Ultimately, we use our results to discuss the relative strengths and weaknesses of DDM compared to MPT. In all cases, however, the MSD data obtained by DDM is nearly indistinguishable from that obtained by MPT, and compares well with bulk rheological data vis-à-vis the generalized Stokes-Einstein relation. Our results show that DDM is a potentially powerful tool to perform probe microrheology experiments in a wider range of fluids than MPT, while circumventing many of its drawbacks.