Oscillatory axial banding of particles suspended in a rotating fluid

In situ syringe rotation system for heavy microparticle suspension stability in electrospinning technique

Electrospinning allows the fabrication of polymeric nonwovens with a wide variety of inclusions in the micro-nanofibers. However, the electrospinning of microparticle-filled polymer solutions is still limited in particle size, density, and concentration, mainly due to suspension instability during the electrospinning process, so it is not commonly investigated despite the vast number of possible applications. In this study, a simple and effective novel rotation device was developed to prevent the settling of microparticles in the polymer solution during electrospinning. The stability of polyvinyl alcohol and polyvinylidene fluoride (PVDF) solutions with indium microparticles (IMPs) of (42 ± 7) μm diameter was evaluated using LASER transmittance inside a syringe, both static and rotating for 24 h. While the static suspensions completely settled at 7 min and 9 h, respectively, depending on solution viscosity, the rotating suspensions remained stable throughout the experiment. The number and distribution of IMPs in PVDF electrospun mats were determined by optic microscopy and a novel x-ray imaging mapping method, showing 165% more IMPs in the mat obtained with the rotating syringe device. A simple analysis of the theoretical background of settling and rotating suspensions was included to understand the working mechanism of the device. Also, the electrospinning of solutions with high loadings of IMPs (up to 400% w/w PVDF) was accomplished. The simplicity and outstanding efficiency of the device shown in this work may serve as a solution to technical difficulties and as an encouragement to future research in microparticle-filled solution electrospinning.

Dynamics of bidensity particle suspensions in a horizontal rotating cylinder

We report Stokesian dynamics simulations of bidensity suspensions rotating in a horizontal cylinder. We studied the phase space and radial and axial patterns in settling as well as floating systems. Each system was composed of particle mixtures of two different densities. As many as eight unique phases are identified for each system along the radial plane. The bidensity system shows similarity to the monodisperse case only when the radial distribution of the particles is completely uniform. Characteristic behavior of the bidensity systems is identical at low rotation rates and contrasting when centrifugal force dominates. Expressing the phase boundaries in terms of dimensionless parameters U_{s}/(ΩR) and g/(Ω^{2}R) gives a linear fit unifying the data in the gravity-dominated regime. At high rotation rates, the behavior is opposing for either system though linear in nature. In the axial direction, number density profiles of both systems affirm the phenomenon of band formation. Location of the axial bands remains the same for heavy and light particles in both systems. We have also reestablished that an inhomogeneous particle configuration in the radial plane induces growing instabilities in the axial plane which amplify to form particle bands similar to monodisperse suspensions.

Transitions between multiple attractors in a granular experiment

We report on a granular experiment that produces multiple quasiperiodic pattern solutions in a rotating flat container filled with a bidisperse mixture of grains. The observed dynamics show a pronounced spatiotemporally periodic drift of segregated patterns. In the course of long experiment durations, the system switches between different states that appear to be stable oscillatory solutions over many periods. In the long-term limit, the system does not tend toward a stationary state. The results complement and extend previous observations in cylindrical geometries, and represent a challenge for modeling and theoretical description.

Formation and stability of band patterns in a rotating suspension-filled cylinder

We explore the phenomenon of segregation and pattern formation in the complex system of a rotating horizontal cylinder completely filled with a dilute suspension of non-Brownian particles. A general dimensionless analysis is presented, which reveals the importance of the different dimensionless parameters involved. A detailed account of the mechanism of segregation and formation of axial bands for the case of low viscosity fluids is given. According to the analysis the axial pressure gradient associated with an inertial-mode excitation within the bounded fluid is responsible for the formation of bands in interleaving nodal planes of the excitation. The question of stability of the band patterns is addressed and a phase diagram in the appropriate dimensionless space is presented.

Banding of suspended particles in a rotating fluid-filled horizontal cylinder

Non-Brownian particles suspended at low volume concentration in a rotating horizontal cylinder filled with a low-viscosity fluid are observed to segregate into well-defined periodic axial bands. We present an experimental investigation of the dependence of the phenomenon on particle characteristics, tube diameter and length, and fluid viscosity. A theoretical explanation of the phenomenon is suggested, in which the segregation occurs as a result of mutual interaction between the particles and inertial waves excited in the bounded fluid. This leads to the result that macroscopic suspended particles accumulate in alternate nodes of the wave excitation, which is in agreement with the experiments, and leads to two degenerate band patterns for each mode. Under some conditions the observed pattern oscillates between the two possible band configurations. The mechanism underlying the oscillations is unclear. A confirmation of the theoretical approach was obtained by means of a photographic capture of the flow field resulting from the inertial waves.

Concentration and velocity patterns in a horizontal rotating suspension of non-Brownian settling particles

We report a systematic experimental study of concentration and velocity patterns formed in a horizontal rotating cylinder filled completely with a monodisperse suspension of non-Brownian settling particles. The system shows a series of concentration and velocity patterns, or phases, with varying rotation rate and solvent viscosity. Individual phases are studied using both side and cross-sectional imaging to examine the detailed flow structures. The overall phase diagram of the system is mapped out as a function of the rotation rate and solvent viscosity. Attempts are made to analyze the functional form of the phase boundaries in order to understand the transition mechanism between different phases.