Dynamics of non-Brownian fiber suspensions under periodic shear

Abstract

We report experiments studying the dynamics of dense non-Brownian fiber suspensions subjected to periodic oscillatory shear. We find that periodic shear initially causes fibers to collide and to undergo irreversible diffusion. As time progresses, the fibers tend to orient in the vorticity direction while the number of collisions decreases. Ultimately, the system goes to one of two steady states: an absorbing steady state, where collisions cease and the fibers undergo reversible trajectories; an active state, where fibers continue to collide causing them to diffuse and undergo irreversible trajectories. Collisions between fibers can be characterized by an effective volume fraction Φ with a critical volume fraction Φc that separates absorbing from active (diffusing) steady states. The effective volume fraction Φ depends on the mean fiber orientation and thus decreases in time as fibers progressively orient under periodic shear. In the limit that the temporal evolution of Φ is slow compared to the activity relaxation time τ, all the data for all strain amplitudes and all concentrations can be scaled onto a single master curve with a functional dependence well-described by t(-β/ν)R(e(-t)R), where tR is the rescaled time. As Φ → Φc, τ diverges. Therefore, for experiments in which Φ(t) starts above Φc but goes to a steady state below Φc, departures from scaling are observed for Φ very near Φc. The critical exponents are measured to be β = 0.84 ± 0.04 and ν = 1.1 ± 0.1, which is consistent with the Manna universality class for directed percolation.

Advances in the Rheological Characterization of Slurries of Elongated Particles

“…Note that the simulated values of S θ quantitatively matched the experimental results over a wide range of conditions (Strednak et al, 2021). Franceschini et al (2014) suggested that the transition in S θ and η'' is controlled by a critical effective volume fraction (Φ c ) of the suspension. The effective volume fraction of a fiber suspension is the ratio of the volume swept out by all fibers during a cycle to the total volume of the suspension, and is therefore proportional to the strain amplitude γ 0 .…”

“…9a shows a strong qualitative difference for the latter case, where η'' (i.e. imaginary component of the complex viscosity) vanishes with increasing oscillation number (N) or remains finite, depending on the strain amplitude γ 0 of the oscillation (Franceschini et al, 2014). The rheological data reported is for aspect ratio A = 11 and a concentration of ϕ = 0.20; importantly, the suspension was highly confined (H/L = 1.5).…”

“…9 Oscillatory rheology and orientation distribution for suspensions of rigid rods at a confinement of H = 1.5L. a) The value of η '' as a function of oscillation number N at A = 11 and ϕ = 0.20 for a range of strain amplitudes γ 0 (Franceschini et al, 2014). The result for γ 0 < 2.60 marks a transition between vanishing and persistent values for η''.…”

“…The result for γ 0 < 2.60 marks a transition between vanishing and persistent values for η''. b) Simulated predictions (Strednak et al, 2021) and an experimental measurement (Franceschini et al, 2014) of the microstructure S θ at steady state as a function of the rescaled strain amplitude, γ 0 ϕ/Φ c . A value of 0.4 was used for Φ c .…”

Actuating Soft Matter with Magnetic Torque

“…The tumbling method of magnetic locomotion relies on the hydrodynamic interaction between the particle(s) and a nearby wall. Nonslip conditions at the surface of a rotating particle assembly and at the surface of a static wall create high shear in the solvent layer between the assembly and the surface . Because of this slipping between fluid layers, magnetic walkers generally show reduced translation motion relative to their rotational speed.…”