Free surface electrospinning of fibers containing microparticles.

Many materials have been fabricated using electrospinning, including pharmaceutical formulations, superhydrophobic surfaces, catalysis supports, filters, and tissue engineering scaffolds. Often these materials can benefit from microparticles included within the electrospun fibers. In this work, we evaluate a high-throughput free surface electrospinning technique to prepare fibers containing microparticles. We investigate the spinnability of polyvinylpyrrolidone (PVP) solutions containing suspended polystyrene (PS) beads of 1, 3, 5, and 10 µm diameter in order to better understand free surface electrospinning of particle suspensions. PS bead suspensions with both 55 kDa PVP and 1.3 MDa PVP were spinnable at 1:10, 1:5, and 1:2 PS:PVP mass loadings for all particle sizes studied. The final average fiber diameters ranged from 0.47 to 1.2 µm and were independent of the particle size and particle loading, indicating that the fiber diameter can be smaller than the particles entrained and can furthermore be adjusted based on solution properties and electrospinning parameters, as is the case for electrospinning of solutions without particles.

Charge segregation depends on particle size in triboelectrically charged granular materials.

Experiments are carried out to examine triboelectric charging in granular systems composed of particles that are chemically identical but differ in size. A methodology is developed so that only particle-particle interactions (but not particle-wall interactions) contribute to the charging. Since all particles are chemically identical, there is no apparent driving force for charge transfer, but charging occurs nonetheless, such that smaller particles tend to charge negatively while larger particles tend to charge positively. For bimodal systems, a model for the frequency of collisions of particles with different size predicts the concentrations for which the observed charge segregation is maximized.

Contact angles of submillimeter particles: connecting wettability to nanoscale surface topography.

A method is presented for the measurement of contact angles of particles in the size range 100-1000 microm, using an optical microscope. This method is used to characterize the wettability of polyethylene particles produced in a gas-phase polymerization process. The as-polymerized particles are shown to be significantly more hydrophobic than melt-processed polyethylene materials. The surface structure of the particles is examined with scanning electron microscopy, and the increased hydrophobicity of the as-polymerized particles is attributed to the nanoscale texture of the particle surfaces.