Dam Break Wave of Thixotropic Fluid

Thixotropy is the characteristic of a fluid to form a gelled structure over time when it is not subjected to shearing, and to liquefy when agitated. Thixotropic fluids are commonly used in the construction industry (e.g., liquid concrete and drilling fluids), and related applications include some forms of mud flows and debris flows. This paper describes a basic study of dam break wave with thixotropic fluid. Theoretical considerations were developed based upon a kinematic wave approximation of the Saint-Venant equations down a prismatic sloping channel. A very simple thixotropic model, which predicts the basic rheological trends of such fluids, was used. It describes the instantaneous state of fluid structure by a single parameter. The analytical solution of the basic flow motion and rheology equations predicts three basic flow regimes depending upon the fluid properties and flow conditions, including the initial “degree of jamming” of the fluid (related to its time of restructuration at rest). These findings were successfully compared with systematic bentonite suspension experiments. The present work is the first theoretical analysis combining the basic principles of unsteady flow motion with a thixotropic fluid model and systematic laboratory experiments.     

How water retention in porous media with cellulose ethers works

Cellulose ethers (CE) are widely used in mortars as water retaining agents; however the cause of retention remains unknown. This paper attempts to clarify the involved mechanisms using several macroscopic experiments (water retention WR standard tests, imbibition and filtration tests). We highlight that WR is not specific to cement but occurs for every porous media. Then, using model porous media and different fluids we point out a critical mechanism of WR with CE solutions: a jamming effect during water transport through the porous medium. This effect may be explained by the presence of polydisperse aggregates which result from native cellulose or hydrophobic interactions. Finally we present a statistical model for filtration which predicts all the qualitative trends observed experimentally.     

Correlation between L-box test and rheological parameters of a homogeneous yield stress fluid

In this paper, a theoretical analysis of the L-box test is proposed in the case where no heterogeneity is induced by the flow. It is first demonstrated that if the standard procedure is followed and the L-box gate promptly lifted, the flow is dominated by inertia effects, depending on the lifting speed of the gate and thus on the operator. When the gate is slowly lifted, the flow and flow stoppage of a homogeneous yield stress fluid in a bounded channel are first studied. The obtained theoretical shape of the sample at stoppage is successfully correlated to the experimental results in the case of limestone powder suspensions. Then, the influence of steel bars was included in both the theoretical and the experimental study. Finally, practical applications of the present work to the case of real self-compacting concrete are suggested.     

Fractional derivative models for ultrasonic characterization of polymer and breast tissue viscoelasticity

The viscoelastic response of hydropolymers, which include glandular breast tissues, may be accurately characterized for some applications with as few as 3 rheological parameters by applying the Kelvin-Voigt fractional derivative (KVFD) modeling approach. We describe a technique for ultrasonic imaging of KVFD parameters in media undergoing unconfined, quasi-static, uniaxial compression. We analyze the KVFD parameter values in simulated and experimental echo data acquired from phantoms and show that the KVFD parameters may concisely characterize the viscoelastic properties of hydropolymers. We then interpret the KVFD parameter values for normal and cancerous breast tissues and hypothesize that this modeling approach may ultimately be applied to tumor differentiation.     

Investigation of intermodulation in acoustic-surface-wave filter

Experimental measurements of the intermodulation of two signals in an acoustic-surface-wave filter are reported. Different cases are presented giving the inband and outband intermodulations for various input power levels from 0 to ?15 dBm at intermediate frequencies near 36 MHz.     

Water transfers within Hemp Lime Concrete followed by NMR

The water transfers between the different phases of a Hemp Lime Concrete (HLC) were followed by NMR measurements. Due to the very different values of the NMR relaxation times of water contained in the different phases of the material the respective amounts of water within the hemp and in the mineral binder can be determined from an analysis of the time evolution of the distribution of T₁ relaxation time of the whole sample. Reliable quantitative estimations in the different phases were obtained, as confirmed by the agreement between the total mass deduced from NMR and the weighted mass at any time. It is shown that for a sealed sample a significant water amount is initially absorbed in the hemp then progressively migrates towards the binder, which supplies additional water for the hydration reactions. For an open (drying) sample the same process occurs but a significant amount of water evaporates, and the hemp dries more rapidly than binder.     

Jamming of cellulose ether solutions in porous medium

The flow of aqueous cellulose ether solutions through a bead packing is investigated using magnetic resonance imaging and filtration measurements. A rather complex behavior dominated by jamming (clogging) and unjamming phenomena in time is observed. With the help of several characterization techniques (laser grain sizing, dynamic light scattering, optical microscopy, and rheometry), we find that the particular methyl(hydroxyethyl) cellulose prepared with a specific protocol, tends to form aggregates in water, even at the lowest achievable concentration. These aggregates are highly polydisperse, ranging from 100 nm to 100 μm in size, and are deformable. Their origin appears to be the hydrophobic links among molecules and the related local crystallization. It is suggested that these features play a key role in the observed jamming/unjamming during filtration tests. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3923–3935, 2015     

Steady state flow of cement suspensions: A micromechanical state of the art

Fresh cementitious pastes can be viewed as suspensions of particles of many different sizes (from several tens of nm to 100 μm) in a continuous fluid phase. This broad poly-dispersity implies that various interactions such as surface forces (or colloidal interactions), Brownian forces, hydrodynamic forces or various contact forces between particles interplay. Depending on the volume fraction of the particles in the mixture, the use of admixtures or the magnitude of either the applied stress or strain rate, one or several of these interactions dominate. Our objective here is not to quantitatively predict rheology of cement pastes but rather to understand and classify the situations where, depending on composition and processing, one or other of the physical phenomena will control the macroscopic behavior. The result of this analysis is a conceptual diagram of predominant interactions in flowing cementitious suspensions under simple shear, as a function of shear rate and solid fraction.     

Pore size NMR imaging

Magnetic resonance imaging (MRI) experiments with industrial or civil engineering materials do not in general aim at determining more than water density distribution or profile. Nevertheless, it is fundamental to know more about their internal structure in relation to their macroscopic behaviour. Two techniques make possible such studies. They involve magnetic resonance imaging coupled with an analysis of respectively freezing and relaxation effects. These techniques do not have the usual magnetic resonance imaging resolution limitations. We present a mathematical treatment of data obtained by these techniques that directly provides spatial maps of the different moments of the pore size distribution.