Extended Fluctuation-Dissipation Theorem for Driven Colloidal Suspensions

• Applicants: Professor Dr. Udo Seifert; Professor Dr. Thomas Speck
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: from 2008 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 60433802

• Final Report Year: 2018

Final Report Abstract

In the second funding period, we have studied the response of a colloidal probe particle to an external force. Such active microrheology has become a valuable tool to probe biological materials. In the analytic approach, we consider the interactions of the probe with “bath” particles but neglect interactions between these bath particles as is appropriate for soft materials (e.g., polymer suspensions). We corroborate results from solving the resulting partial differential equation with direct numerical simulations. Our central result is the emergence of force thinning, a novel mechanism in which a strongly driven, attractive probe discontinuously reduces the viscosity of the medium. In a second step, we have considered the influence of a second, quiescent probe on the driven probe. Moreover, we have investigated related problems in stochastic thermodynamics: the effect of coarsegraining a density field on the fluctuation theorem and the extraction of work from a driven bath, which have led to further publications.

Publications

• Effective confinement as origin of the equivalence of kinetic temperature and fluctuationdissipation ratio in a dense shear-driven suspension. Phys. Rev. E 85, 021103 (2012)
  B. Lander, U. Seifert, and T. Speck
  
• Crystallization in a sheared colloidal suspension. J. Chem. Phys. 138, 224907 (2013)
  B. Lander, U. Seifert, and T. Speck
  
• Stochastic thermodynamics of fluctuating density fields: Non-equilibrium free energy differences under coarse-graining. J. Chem. Phys. 139, 204109 (2013)
  T. Leonard, B. Lander, U. Seifert, and T. Speck
  
• Discontinuous thinning in active microrheology of soft complex matter. Phys. Rev. E 94, 062610 (2016)
  R. Wulfert, U. Seifert, and T. Speck
  
• Driven Brownian particle as a paradigm for a nonequilibrium heat bath: Effective temperature and cyclic work extraction. Phys. Rev. E 95, 050103 (2017)
  R. Wulfert, M. Oechsle, T. Speck, and U. Seifert
  
• Nonequilibrium depletion interactions in active microrheology. Soft Matter 13, 9093 (2017)
  R. Wulfert, U. Seifert, and T. Speck