Decreased Crystal Plasticity by Solvation – the Case of Levofloxacin Acesulfame

Molecular Pharmaceutics · 2025-02-06 · 1 citations

A correlation between crystal packing efficiency and plasticity can be used to efficiently rank the order of plasticity of crystal forms of a compound from their crystal structures. It was hypothesized that a lower crystal packing efficiency and density lead to higher plasticity of a hydrate. This hypothesis was only supported by the observation that stoichiometric hydrates exhibit both higher plasticity and lower packing efficiency than the corresponding anhydrates. In this work, we have tested this hypothesis using a channeled hemimethanol solvate of a levofloxacin acesulfame salt. Our results strengthen the hypothesis since, compared to the isostructural anhydrate, the hemimethanol solvate exhibits both higher packing efficiency and lower plasticity. The lower plasticity of the solvate is confirmed by crystal structure analysis and energy framework calculations. If this correlation between crystal packing efficiency and plasticity is robust, then we can objectively predict material plasticity of structurally related crystals based on crystal packing efficiency.

Lift-off of a single particle in Newtonian and viscoelastic fluids by direct numerical simulation

Journal of Fluid Mechanics · 2001-07-05 · 113 citations

In this paper we study the lift-off to equilibrium of a single circular particle in Newtonian and viscoelastic fluids by direct numerical simulation. A particle heavier than the fluid is driven forward on the bottom of a channel by a plane Poiseuille flow. After a certain critical Reynolds number, the particle rises from the wall to an equilibrium height at which the buoyant weight just balances the upward thrust from the hydrodynamic force. The aim of the calculation is the determination of the critical lift-off condition and the evolution of the height, velocity and angular velocity of the particle as a function of the pressure gradient and material and geometric parameters. The critical Reynolds number for lift-off is found to be larger for a heavier particle whereas it is lower for a particle in a viscoelastic fluid. A correlation for the critical shear Reynolds number for lift-off is obtained. The equilibrium height increases with the Reynolds number, the fluid elasticity and the slip angular velocity of the particle. Simulations of single particle lift-off at higher Reynolds numbers in a Newtonian fluid by Choi & Joseph (2001) but reported here show multiple steady states and hysteresis loops. This is shown here to be due to the presence of two turning points of the equilibrium solution.

Normal Stresses on the Surface of a Rigid Body in an Oldroyd-B Fluid

Journal of Fluids Engineering · 2001-08-23 · 4 citations

In this note we present a proof showing that the contribution from the extra stress tensor to the normal component of the stress on the surface of a moving rigid body in an incompressible Oldroyd-B fluid is zero.

Effects of shear thinning on migration of neutrally buoyant particles in pressure driven flow of Newtonian and viscoelastic fluids

Journal of Non-Newtonian Fluid Mechanics · 2000-05-01 · 66 citations

Direct simulation of the sedimentation of elliptic particles in Oldroyd-B fluids

Journal of Fluid Mechanics · 1998-05-10 · 112 citations

Cross-stream migration and stable orientations of elliptic particles falling in an Oldroyd-B fluid in a channel are studied. We show that the normal component of the extra stress on a rigid body vanishes; lateral forces and torques are determined by the pressure. Inertia turns the long side of the ellipse across the stream and elasticity turns it along the stream; tilted off-centre falling is unstable. There are two critical numbers: the elasticity and Mach numbers. When the elasticity number is smaller than critical the fluid is essentially Newtonian with broadside-on falling at the centreline of the channel. For larger elasticity numbers the settling turns the long side of the particle along the stream in the channel centre for all velocities below a critical one, identified with a critical Mach number of order one. For larger Mach numbers the ellipse flips into broadside-on falling again. The critical numbers are functions of the channel blockage ratio, the particle aspect ratio and the retardation/relaxation time ratio of the fluid. Two ellipses falling near to each other, attract, line-up vertically and straighten-out with long sides vertical. Stable, off-centre tilting is found for ellipses falling in shear-thinning fluids and for cylinders with flat ends in which particles tend to align their longest diameter with gravity.

Direct simulation of the motion of solid particles in Couette and Poiseuille flows of viscoelastic fluids

Journal of Fluid Mechanics · 1997-07-25 · 134 citations

This paper reports the results of direct numerical simulation of the motion of a two-dimensional circular cylinder in Couette flow and in Poiseuille flow of an Oldroyd-B fluid. Both neutrally buoyant and non-neutrally buoyant cylinders are considered. The cylinder's motion and the mechanisms which cause the cylinders to migrate are studied. The stable equilibrium position of neutrally buoyant particles varies with inertia, elasticity, shear thinning and the blockage ratio of the channel in both shear flows. Shear thinning promotes the migration of the cylinder to the wall while inertia causes the cylinder to migrate away from the wall. The cylinder moves closer to the wall in a narrower channel. In a Poiseuille flow, the effect of elastic normal stresses is manifested by an attraction toward the nearby wall if the blockage is strong. If the blockage is weak, the normal stresses act through the curvature of the inflow velocity profile and generate a lateral force that points to the centreline. In both cases, the migration of particles is controlled by elastic normal stresses which in the limit of slow flow in two dimensions are compressive and proportional to the square of the shear rate on the body. A slightly buoyant cylinder in Couette flow migrates to an equilibrium position nearer the centreline of the channel in a viscoelastic fluid than in a Newtonian fluid. On the other hand, the same slightly buoyant cylinder in Poiseuille flow moves to a stable position farther away from the centreline of the channel in a viscoelastic fluid than in a Newtonian fluid. Marked effects of shear thinning are documented and discussed.

The Motion and Interaction of Solid Particles in Viscoelastic Liquids

1996-11-17

Abstract In this paper we present numerical and experimental results on the motion and interaction of solid particles in polymeric fluids. The two-dimensional numerical work investigates the viscoelastic effects on the sedimentation of a particle in the presence of solid walls or another particle. The Navier-Stokes equations coupled with an Oldroyd-B model are solved using a finite element method, and the particles are moved according to their equations of motion. In a vertical channel, a particle settling close to one side wall experiences a repulsion from the wall; a particle settling farther away from the wall is attracted to it. Two particles settling in tandem attract and form a doublet if their initial separation is not too large. Two particles settling side by side approach each other and the doublet also rotate till the line of centers is aligned with the direction of fall. The experimental part studies the behavior of single particles and suspensions in polymer solutions in a torsional flow. Four issues are investigated: the radial migration of a spherical particle, the rotation and migration of a cylindrical rod, the particle-particle interaction and microstructures in a suspension of spheres and the microstructures in a suspension of rods. A spherical particle migrates outward at a constant velocity unless the polymer solution is very dilute. A rod has two modes of motion depending on its shape, initial orientation, the local shear rate and the magnitude of normal stresses in the fluid. When a suspension is sheared, spheres form chains along the flow direction. These chains may connect and form circular rings, which migrate outward at a velocity much higher than that for a single sphere. Rods interact with each other and aggregate in much the same way, but to a less extent than spheres. Particle interaction and aggregation can be explained by two fundamental mechanisms discovered in the numerical simulations of sedimentation.

Dynamic simulation of sedimentation of solid particles in an Oldroyd-B fluid

Journal of Non-Newtonian Fluid Mechanics · 1996-03-01 · 87 citations

Wall effects on the flow of viscoelastic fluids around a circular cylinder

Journal of Non-Newtonian Fluid Mechanics · 1995-11-01 · 74 citations

Dynamic simulation of the motion of capsules in pipelines

Journal of Fluid Mechanics · 1995-03-10 · 57 citations

In this paper we report results of two-dimensional simulations of the motion of elliptic capsules carried by a Poiseuille flow in a channel. The numerical method allows computation of the capsule motion and the fluid flow around the capsule, and accurate evaluation of the lift force and torque. Results show that the motion of a capsule which is heavier than the carrying fluid may be decomposed into three stages: initial lift-off, transient oscillations and steady flying. The behaviour of the capsule during initial lift-off and steady flying is analysed by studying the pressure and shear stress distributions on the capsule. The dominant mechanism for the lift force and torque is lubrication or inertia or a combination of the two under different conditions. The lift-off velocity for the ellipse in two dimensions is compared with experimental values for cylindrical capsules in pipes. Finally, the mechanisms of lift for capsules are applied to flying core flows, and it is argued that inertial forces are responsible for levitating heavy crude oil cores lubricated by water in a horizontal pipeline.