Donald L. Feke

Separation of dispersed phases from liquids in acoustically driven chambers

Abstract A separation process for the collection and processing of fine secondary phases in liquids has been developed, in which a stationary acoustic field at ultrasonic frequencies is superimposed on suspensions flowing through filled chambers. The acoustic radiation force acts to drive second-phase inclusions to either the nodes or antinodes of the stationary field, and acts to hold the secondary-phase material in position relative to the bulk liquid flow. Separation devices consisting of broadband lead-zirconate—lead-titanate transducers, bonded to the ends of glass and aluminium tubes (up to 4 cm in diameter and 25 cm in length, and ranging from 0.9 to 2 mm in wall thickness), were constructed. These were driven with continuous a.c. voltages at specific frequencies in the range of 0.35–1.4 MHz to obtain a forced coincidence response in which the flexural vibrations of the tube wall are matched in phase and geometry with higher-order acoustic duct modes in the liquid. This excitation produced stationary fields with peak acoustic pressures of the order of 1 MPa in water. Particles ranging in size from 0.1 to 100 μm have been concentrated in aqueous media into nodal zones with characteristic times of less than 1 s. Chambers with 0.11 volume, driven at frequencies near 625 kHz, use approximately 25 W of electrical power. Collection efficiencies of up to 97.5% have been measured. The trapped particles can be moved to either end of the cell and removed through an exit port by repeatedly sweepping the driving frequency over a known range and period. Applications of these principles include separations from fluid mixtures of fine solids, immiscible liquids, or biological cells.

Fractionation of mixed particulate solids according to compressibility using ultrasonic standing wave fields

Abstract A continuous fractionation method for mixed particulate solids has been developed. When applied to suspensions of solids having substantially equal densities, the separation method distinguishes the particulates on the basis of their compressibility. The technique can be applied to solids ranging from micron to millimeter scale with overlapping or distinct particle size distributions. The method involves suspension of the solids into an appropriate processing medium, the application of a standing acoustic wave field to sort the particles across a separator channel, and the use of laminar flow fields along the channel to accomplish the ultimate separation. An analytical model of the separation process, based on particle trajectories through the separator chamber, is presented to show the sensitivity of the method to system properties and to illustrate its selectivity. Results of prototypical experiments with polymer particles further demonstrate the method.

Methodology for fractionating suspended particles using ultrasonic standing wave and divided flow fields

Abstract A methodology to fractionate suspended particles according to size and/or acoustic properties is reported. This fractionation is accomplished by simultaneously subjecting the particles to a resonant acoustic field and a laminar flow field propagating in an orthogonal direction. The acoustic field induces a redistribution of the particles within the cross-section of a narrow separator channel while the laminar flow transports the particles along the separator channel towards the exit. By altering the strength of the flow relative to the strength of the acoustic field, the desired fractionation can be controlled. Proof-of-principle experiments conducted with binary mixtures of polystyrene particles are reported. The performance of the experimental device was analyzed using a model based on calculations of the trajectory of particles through the chamber. The acoustic method has potential to rapidly and selectively factionate suspended solids with very low specific power consumption.

Analysis of Agglomerate Rupture in Linear Flow Fields

Abstract A dispersive mixing model focusing on the rupture of agglomerates as the step that primarily determines the dynamics of the mixing process was developed and analyzed. Rupture is predicted to occur when hydrodynamic forces exerted on the outer surface of the agglomerate exceed cohesive forces binding the agglomerate together. Agglomerates are modeled as clusters of aggregates bound by van der Waals forces. The magnitude and orientation of the rupturing hydrondynamic force depend on the local stress field in the fluid. Cleavage of the agglomerate is predicted to occur at the mid-plane of the agglomerate where the effects of hydrodynamic tension is the largest. Under the assumption that parent agglomerates and their fragments have the same shape, the kinetics of the rupture process is found to be independent of the absolute size of the agglomerate. Following from this is the result that the dispersion process is governed by flow dynamics, i.e., a minimum value of flow strength, which depends on the geometry of the bulk flow field, is required for rupture of the agglomerate. Agglomerate rupture was investigated in four flow geometries: simple shear; pure elongation; uniaxial extension, and biaxial extension. The efficiency of each flow geometry is compared on the basis of power and time requirements to achieve a given degree of dispersion.

Enhanced synchronized ultrasonic and flow-field fractionation of suspensions

Abstract A fractionation method for fine-particle suspensions based on variations in the speed of response to ultrasonic standing wave fields has been developed. The method is sensitive to a combination of the particle size as well as the particle and suspending fluid density and compressibility. The fractionation is accomplished within a narrow-gap rectangular channel having a solid barrier positioned along its midplane. Ultrasonic standing wave fields of two alternating frequencies are applied across the gap to induce a partial separation at short irradiation times. A co-ordinated bidirectional laminar flow field is used to transform these partial separations into useful separations along the chamber. In comparison with similar fraction strategies that use other types of fields to accomplish the separation, the acoustic forces acting on particles can be quite strong, thereby enabling fast, continuous, and controllable fractionation of micrometre-sized particles. An analytical model based on the trajectories of particles in response to the acoustic and flow cycles was developed. Model predictions indicate how the fractionation can be controlled through the choice of cycle parameters. Experimental results using a 325 mesh polystyrene particle suspension demonstrate that sharp fractionations of nearly neutrally buoyant micrometre-sized particles can be achieved.

Separation devices based on forced coincidence response of fluid‐filled pipes

A separation process based on the acoustic radiation force created in stationary fields produced by a forced coincidence excitation at ultrasonic frequencies of fluid‐filled pipes has been developed. The efficacy of this method for the collection and manipulation of fine secondary phases in flowing suspensions will be compared to equivalent operations in stationary fields generated in acoustic interferometer chambers operated without coincidence effects. The basis for the axial translation of the phases concentrated at the pressure nodes to either end of the cell as a result of an applied periodic sweep in the driving frequency will be examined.

Retention and Viability Characteristics of Mammalian Cells in an Acoustically Driven Polymer Mesh

A processing approach for the collection and retention of mammalian cells within a high porosity polyester mesh having millimeter‐sized pores has been studied. Cell retention occurs via energizing the mesh with a low intensity, resonant acoustic field. The resulting acoustic field induces the interaction of cells with elements of the mesh or with each other and effectively prevents the entrainment of cells in the effluent stream. Experiments involving aqueous suspensions of polystyrene particles were used to provide benchmark data on the performance of the acoustic retention cell. Experiments using mouse hybridoma cells showed that retention densities of over 1.5 × 108 cell/mL could be obtained. In addition, the acoustic field was shown to produce a negligible effect on cell viability for short‐term exposure.

Acoustically driven collection of suspended particles within porous media

A method for collecting fine particles using a porous medium subjected to a standing ultrasonic wave field is described. Proof-of-concept experiments using aqueous suspensions of polystyrene spheres demonstrated the collection of particles up to two orders of magnitude smaller than the size of pores within the porous medium. Two types of porous media were studied; unconsolidated beds formed from glass spheres, and aluminum foam meshes. Removal of particles from the porous media is readily accomplished by deactivating the acoustic field and flushing the particles from the porous medium through continued liquid flow. The effects of processing conditions (suspension flow rate, acoustic power, and feed concentration) on the retention of particles were assessed. Explanations of the mechanism by which the collection occurs are postulated and an assessment of the practicality of the concept for suspension processing is presented.

Mechanical and cell viability properties of crosslinked low- and high-molecular weight poly(ethylene glycol) diacrylate blends

There is a strong need for tissue engineering scaffolds that are mechanically robust, exhibit good biocompatibility, and can be made from readily available materials. To this end, blends of commercially available poly(ethylene glycol) diacrylate (PEGDA) with molecular weights of 400 and 3400 were UV-crosslinked at total polymer concentrations that varied systematically from 20 to 40 wt %. The compressive strength and cell viability were determined for each PEGDA mixture. The compressive modulus of the blends was maximized when the weight percent ratio PEGDA3400/400 was about 40/60, with the compressive strength reaching 1.7 MPa. Cell viability results with a LIVE/DEAD fluorescence assay show an average viability of approximately 80% at a total PEGDA concentration of 20 wt % for all blends. Increasing the total polymer concentration increased the compressive modulus of a polymer, but adversely affected cell viability for all the PEGDA blend compositions. The blend composition affected the mechanical behavior of the discs, where a higher degree of crosslinking was achieved by increasing the concentration of shorter chained PEGDA400, whereas elasticity was gained by incorporating longer chained PEGDA3400 into the blends. These results can be exploited for use in tissue engineering applications, where a mechanically robust scaffold is advantageous.

Dispersion of titanium dioxide agglomerates in viscous media

Abstract The dispersion behavior of titanium dioxide agglomerates in viscous media was studied. Under application of shear within a cone-and-plate device, the titanium dioxide agglomerates primarily dispersed by an erosion process in which small fragments separate from the surface. The cohesiveness of unwetted powder was quantified using a compression test method. In some cases, the shear stress necessary to produce erosion was found to be an order of magnitude smaller than the measured cohesivity. Medium infiltration within agglomerates was assessed through observation of penetration of the medium into powder compacts. The kinetics of the erosion process was highly sensitive to the overall porosity of the agglomerates. In the case of high porosity, the erosion rate depends on the speed of medium infiltration within the agglomerate, and the strength of the applied shear stress. A relatively wide size distribution of fragments was observed. For low-porosity agglomerates, the erosion process depends on the magnitude of shear stress, the cohesive strength of the agglomerates, and agglomerate—medium interactions. A narrower distribution of fragments, having smaller mean size, was observed.