Antti Puisto

Modeling the viscosity and aggregation of suspensions of highly anisotropic nanoparticles

The rheology of nanofiber suspensions is studied solving numerically the Population Balance Equations (PBE). To account for the anisotropic nature of nanofibers, a relation is proposed for their hydrodynamic volume. The suspension viscosity is calculated using the computed aggregate size distributions together with the Krieger-Dougherty constitutive equation. The model is fitted to experimental flow curves for Carbon NanoFibers (CNF) and for NanoFibrillated Cellulose (NFC), giving a first estimation of the microscopic anisotropy parameter, and yielding information on the structural properties and rheology of each system.

Transient shear banding in time-dependent fluids

We study the dynamics of shear-band formation and evolution using a simple rheological model. The description couples the local structure and viscosity to the applied shear stress. We consider in detail the Couette geometry, where the model is solved iteratively with the Navier-Stokes equation to obtain the time evolution of the local velocity and viscosity fields. It is found that the underlying reason for dynamic effects is the nonhomogeneous shear distribution, which is amplified due to a positive feedback between the flow field and the viscosity response of the shear thinning fluid. This offers a simple explanation for the recent observations of transient shear banding in time-dependent fluids. Extensions to more complicated rheological systems are considered.

The vane method and kinetic modeling: shear rheology of nanofibrillated cellulose suspensions

We conduct rheological characterization of nanofibrillated cellulose (NFC) suspensions, a highly non-Newtonian complex fluid, at several concentrations. Special care is taken to cope with the prevalent problems of time scale issues, wall depletion and confinement effects. We do this by combining the wide-gap vane geometry, extremely long measurement times, and modeling. We take into account the wide-gap related stress heterogeneity by extending upon mainstream methods and apply a gap correction. Furthermore, we rationalize the experimental data through a simple viscous structural model. With these tools we find that, owing to the small size of the particles subjected to Brownian motion, the NFC suspensions exhibit a critical shear rate, where the flow curve experiences a turning point. This makes the steady state of these suspensions at low shear rates non-unique. To optimize various mixing and pumping applications, such history dependent tendency of NFC suspensions to shear band needs to be taken into account.

Apparent wall slip in non-Brownian hard-sphere suspensions

We analyze apparent wall slip, the reduction of particle concentration near the wall, in hard-sphere suspensions at concentrations well below the jamming limit utilizing a continuum level diffusion model. The approach extends a constitutive equation proposed earlier with two additional potentials describing the effects of gravitation and wall-particle repulsion. We find that although both mechanisms are shear independent by nature, due to the shear-rate-dependent counter-balancing particle migration fluxes, the resulting net effect is non-linearly shear dependent, causing larger slip at small shear rates. In effect, this shows up in the classically measured flow curves as a mild shear thickening regime at the transition from small to intermediate shear rates.Graphical abstract

Transient shear banding in viscoelastic Maxwell fluids

The fluidization of complex fluids is studied in the context of a Maxwell viscoelastic structural fluid model and compared to the purely viscous case. Solving iteratively the structural models together with the Navier–Stokes equation for the circular Couette flow gives spatially and temporally resolved velocity fields closely resembling those found experimentally for viscoelastic carbopol gels. Namely, transient shear banding is found during the initial fluidization phase. Although both structural models show transient shear bands, the viscoelastic one captures the experimental observations in greater detail, showing, for instance, the elastic backward flows during the transient shear band initialization stage.

Start-up inertia as an origin for heterogeneous flow

For quite some time nonmonotonic flow curve was thought to be a requirement for shear banded flows in complex fluids. Thus, in simple yield stress fluids shear banding was considered to be absent. Recent spatially resolved rheological experiments have found simple yield stress fluids to exhibit shear banded flow profiles. One proposed mechanism for the initiation of such transient shear banding process has been a small stress heterogeneity rising from the experimental device geometry. Here, using computational fluid dynamics methods, we show that transient shear banding can be initialized even under homogeneous stress conditions by the fluid start-up inertia, and that such mechanism indeed is present in realistic experimental conditions.

Foam flows through a local constriction

We present an experimental study of the flow of a liquid foam, composed of a monolayer of millimetric bubbles, forced to invade an inhomogeneous medium at a constant flow rate. To model the simplest heterogeneous fracture medium, we use a Hele-Shaw cell consisting of two glass plates separated by a millimetric gap, with a local constriction. This single defect localized in the middle of the cell reduces locally its gap thickness, and thus its local permeability. We investigate here the influence of the geometrical property of the defect, specifically its height, on the average steady-state flow of the foam. In the frame of the flowing foam, we can observe a clear recirculation around the obstacle, characterized by a quadrupolar velocity field with a negative wake downstream the obstacle, which intensity evolves systematically with the obstacle height.