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| 2. |
- Bouzid, Mehdi, et al.
(författare)
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Non-local rheology of dense granular flows
- 2017
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2101-6275 .- 2100-014X. ; 140
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Tidskriftsartikel (refereegranskat)abstract
- The rheology of dense granular flows is studied numerically in a shear cell controlled at constant pressure and shear stress, confined between two granular shear flows. We show that a liquid state can be achieved even far below the yield stress, whose flow can be described with the same rheology as above the yield stress. A non-local constitutive relation is derived from dimensional analysis through a gradient expansion and calibrated using the spatial relaxation of velocity profiles observed under homogeneous stresses. Both for frictional and frictionless grains, the relaxation length is found to diverge as the inverse square root of the distance to the yield point, on both sides of that point. We also make use of a micro-rheometer to determine the influence of a distant shear band on the local rheological behaviour. Finally, we compare various approaches based on different non-local constitutive relations and choices for the fluidity parameter. We emphasise that, to discriminate between the different approaches proposed in the literature, one has to go beyond the predictions derived from linearisation around a uniform stress profile, such as that obtained in a simple shear cell. We argue that future tests can be based on the nature of the chosen fluidity parameter, and the related boundary conditions, as well as the hypothesis made to derive the models and the dynamical mechanisms underlying their dynamics.
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| 3. |
- Dong, Junhao, et al.
(författare)
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Analog of discontinuous shear thickening flows under confining pressure
- 2017
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Ingår i: Physical Review Fluids. - 2469-990X. ; 2:8
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Tidskriftsartikel (refereegranskat)abstract
- We use two-dimensional numerical simulations to study dense suspensions of non-Brownian hard particles using the Critical Load Model (CLM) under constant confining pressures. At constant packing fraction this simple model shows shear thickening as the tangential forces get activated upon increased shear stresses. By parameterizing a simple binary system of frictional and nonfrictional particles of different proportions we show that the jamming packing fraction, at which the viscosity diverges, is controlled by the fraction of frictional contacts. The viscosity of dense suspensions can thereby be expressed as a function of the fraction of frictional contacts as well as the packing fraction of solid particles. In addition, we show that there exists a simple relationship between the fraction of frictional contacts and the two control parameters (under confining pressure): the viscous number J and the ratio between the repulsive barrier force and confining pressure. Under confining pressures the viscosity curves are found to depend on the shear protocol, with the possibility of yielding negative dynamic compressibility, an analog to the discontinous shear thickening found at high but constant packing fractions for the same system.
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| 4. |
- Dong, Junhao, et al.
(författare)
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Transition from steady shear to oscillatory shear rheology of dense suspensions
- 2020
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Ingår i: Physical Review E. - 2470-0045. ; 102:5
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Tidskriftsartikel (refereegranskat)abstract
- Recent studies have highlighted that oscillatory and time-dependent shear flows might help increase the flowability of dense suspensions. While most focus has been on cross-flows we here study a simple two-dimensional suspensions where we apply simultaneously oscillatory and stationary shear along the same direction. We first show that the dissipative viscosities in this set-up significantly decrease with an increasing shear-rate magnitude of the oscillations and given that the oscillatory strain is small, in a similar fashion as found previously for cross-flow oscillations. As for cross-flow oscillations, the decrease can be attributed to the large decrease in the number of contacts and an altered microstructure as one transitions from a steady shear to an oscillatory shear dominated rheology. As subresults we find both an extension to the μ(J) rheology, a constitutive relationship between the shear stresses and the shear rate, valid for oscillatory shear flows and that shear-jamming of frictional particles at oscillatory shear dominated flows occurs at higher packing fractions compared to steady shear dominated flows.
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| 5. |
- Dong, Junhao, et al.
(författare)
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Unifying viscous and inertial regimes of discontinuous shear thickening suspensions
- 2020
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Ingår i: Journal of Rheology. - : Society of Rheology. - 0148-6055 .- 1520-8516. ; 64:2, s. 255-266
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Tidskriftsartikel (refereegranskat)abstract
- The rheology of dense suspensions in the viscous regime can be characterized by the viscous number J and granular flows by the inertia number I, both relating the shear rate γ - to the confining pressure P. Furthermore, several works have shown that the rheology of suspensions and granular flows can be unified, and this unification is used to characterize suspensions with non-negligible particle inertia from the viscous to the inertial regime (i.e., granular flows). There have also been recent works that apply this unification to suspensions where the number of frictional contacts increases upon the shear rate with constant packing fraction, a model that has successfully described discontinuous shear thickening in suspension flows. We and other researchers have previously shown that the fraction of frictional contacts χ f is a key control parameter for friction-driven shear thickening in the viscous regime. It is, however, difficult to control and study the effect of the number of frictional contacts on the rheology of dense suspensions at constant packing fraction as this quantity increases sharply around a threshold shear rate. In this work, we extend our previous work and use numerical simulations to study particle flows in both the viscous and inertial regimes as well as for suspensions in the crossover between these regimes with varying χ f. By having pressure-controlled simulations, we are able to keep χ f at values between 0 and 1, hence critically testing the validity of a unification at intermediate values. With the help of a binary model composed of nonfrictional and frictional particles, where χ f is well-controlled, we manage to obtain expressions for constitutive laws in both limits. For the critical load model, we find a simple relationship between χ f and the pressure-rescaled threshold force f applicable for both the viscous and inertial regimes. Combining these expressions, we then show that suspensions in the crossover between these regimes can be characterized by four dimensionless numbers, f, K, K μ, and K z, where the last three are simple combinations of J and I and verified against numerical simulations. These expressions can be further simplified by approximating K μ = K, which we show is a good approximation close to shear jamming or at low Stokes numbers. In the end, we construct a dimensionless parameter encoding various shear protocols and show predictions of behaviors of suspensions under different shear conditions. Having derived constitutive relations from constant pressure simulations, we finally test our relations under constant volume assumption which indeed well captures the various discontinuous shear thickening behaviors seen at different Stokes numbers. Finally, we discuss the role of having a varying microscopic friction coefficient μ p as a function of the normal force.
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| 6. |
- Forsman, Jan, et al.
(författare)
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A classical density functional theory of ionic liquids.
- 2011
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 115:16, s. 4606-4612
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Tidskriftsartikel (refereegranskat)abstract
- We present a simple, classical density functional approach to the study of simple models of room temperature ionic liquids. Dispersion attractions as well as ion correlation effects and excluded volume packing are taken into account. The oligomeric structure, common to many ionic liquid molecules, is handled by a polymer density functional treatment. The theory is evaluated by comparisons with simulations, with an emphasis on the differential capacitance, an experimentally measurable quantity of significant practical interest.
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| 7. |
- Kanduc, M., et al.
(författare)
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Weak- and strong-coupling electrostatic interactions between asymmetrically charged planar surfaces
- 2008
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Ingår i: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics). - 1539-3755. ; 78:6
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Tidskriftsartikel (refereegranskat)abstract
- We compare weak- and strong-coupling theory of counterion-mediated electrostatic interactions between two asymmetrically charged plates with extensive Monte Carlo simulations. Analytical results in both weak- and strong-coupling limits compare excellently with simulations in their respective regimes of validity. The system shows a surprisingly rich structure in terms of interactions between the surfaces as well as fundamental qualitative differences in behavior in the weak- and the strong-coupling limits.
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| 8. |
- Lund, Mikael, et al.
(författare)
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FAUNUS: An object oriented framework for molecular simulation
- 2008
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Ingår i: Source Code for Biology and Medicine. - : Springer Science and Business Media LLC. - 1751-0473. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: We present a C++ class library for Monte Carlo simulation of molecular systems, including proteins in solution. The design is generic and highly modular, enabling multiple developers to easily implement additional features. The statistical mechanical methods are documented by extensive use of code comments that - subsequently - are collected to automatically build a web-based manual. RESULTS: We show how an object oriented design can be used to create an intuitively appealing coding framework for molecular simulation. This is exemplified in a minimalistic C++ program that can calculate protein protonation states. We further discuss performance issues related to high level coding abstraction. CONCLUSION: C++ and the Standard Template Library (STL) provide a high-performance platform for generic molecular modeling. Automatic generation of code documentation from inline comments has proven particularly useful in that no separate manual needs to be maintained.
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| 9. |
- Palaia, Ivan, et al.
(författare)
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A correlation-hole approach to the electric double layer with counter-ions only
- 2018
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Ingår i: Molecular Physics. - : Informa UK Limited. - 1362-3028 .- 0026-8976. ; 116:21-22, s. 3134-3146
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Tidskriftsartikel (refereegranskat)abstract
- We study a classical system of identically charged counter-ions near a planar wall carrying a uniform surface charge density. The equilibrium statistical mechanics of the system depends on a single dimensionless coupling parameter. A new self-consistent theory of the correlation-hole type is proposed which leads to a modified Poisson–Boltzmann integral equation for the density profile, convenient for analytical progress and straightforward to solve numerically. The exact density profiles are recovered in the limits of weak and strong couplings. In contrast to previous theoretical attempts of the test-charge family, the density profiles fulfil the contact-value theorem at all values of the coupling constant and exhibit the mean-field decay at asymptotically large distances from the wall, as expected. We furthermore show that the density corrections at large couplings exhibit the proper dependence on coupling parameter and distance to the charged wall. The numerical results for intermediate values of the coupling provide accurate density profiles which are in good agreement with those obtained by Monte Carlo simulations. The crossover to mean-field behaviour at large distance is studied in detail.
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| 10. |
- Popa, Ionel, et al.
(författare)
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Long-Ranged Attractive Forces Induced by Adsorbed Dendrimers: Direct Force Measurements and Computer Simulations
- 2009
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 25:21, s. 12435-12438
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Tidskriftsartikel (refereegranskat)abstract
- Interaction forces between charged interfaces in the presence of oppositely charged dendrimers are studied by experiment and simulation. The experiments involve direct force measurements with all atomic force microscope (AFM) between two negatively charged colloidal particles in the presence of adsorbed, positively charged globular dendrimers. The simulations are carried out by treating the macroions explicitly, while the small salt ions are treated implicitly through the Debye-Huckel approximation. The system undergoes overcharging, and at the isoelectric point long-ranged attractive electrostatic forces are present. The range of the attraction is on the order of half the Debye length at high salt concentration, but it becomes smaller at low salt concentration. Away from the isoelectric point, repulsive electrostatic forces are observed due to diffuse layer overlap. A semiquantitative agreement between experiment and Simulation is obtained, despite the fact that the simple theoretical model does not involve any adjustable parameters. This study provides for the first time detailed comparison between experimental and simulation data of interaction forces between colloidal particles in the presence of multivalent macroions and monovalent salt ions.
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