Simultaneous Absolute Determination of Particle Size and Effective Density of Submicron Colloids by Disc Centrifuge Photosedimentometry

Disc centrifuge photosedimentometry (DCP) with fluids of different densities is used to simultaneously determine the particle size and effective density of spherical silica particles. Incorporation of a calibrated infrared pyrometer into a DCP instrument is shown to enhance the measurement capability of the DCP technique by correcting for the temperature dependence of the spin fluid’s density and viscosity. Advantages of absolute DCP determinations for size and density analysis relative to standardized DCP measurements include the elimination of instrument standardization with a particle of known density and measurements or estimation of the effective particle density. The reliability of diameter determinations provided by absolute DCP was confirmed using silica particles with nominal diameters ranging from 250 to 700 nm by comparison of these analyses with a diameter determination by transmission electron microscopy for silica particle size standards. Effective densities determined by absolute DCP for the silica particles ranged from 2.02 to 2.34 g/cm³. These findings indicate that the silica particles have little or no porosity. The reported characterization of colloidal silica using absolute DCP suggests applicability of the technique to a variety of particle types including colloidal materials other than silica, core–shell particles, compositionally heterogeneous mixtures of nanoparticles, and irregularly shaped, structured colloids

Nanoscopic Vibrations of Bacteria with Different Cell-Wall Properties Adhering to Surfaces under Flow and Static Conditions

Bacteria adhering to surfaces demonstrate random, nanoscopic vibrations around their equilibrium positions. This paper compares vibrational amplitudes of bacteria adhering to glass. Spring constants of the bond are derived from vibrational amplitudes and related to the electrophoretic softness of the cell surfaces and dissipation shifts measured upon bacterial adhesion in a quartz-crystal-microbalance (QCM-D). Experiments were conducted with six bacterial strains with pairwise differences in cell surface characteristics. Vibrational amplitudes were highest in low ionic strength suspensions. Under fluid flow, vibrational amplitudes were lower in the direction of flow than perpendicular to it because stretching of cell surface polymers in the direction of flow causes stiffening of the polyelectrolyte network surrounding a bacterium. Under static conditions (0.57 mM), vibrational amplitudes of fibrillated Streptococcus salivarius HB7 (145 nm) were higher than that of a bald mutant HB-C12 (76 nm). Amplitudes of moderately extracellular polymeric substance (EPS) producing Staphylococcus epidermidis ATCC35983 (47 nm) were more than twice the amplitudes of strongly EPS producing S. epidermidis ATCC35984 (21 nm). No differences were found between Staphylococcus aureus strains differing in membrane cross-linking. High vibrational amplitudes corresponded with low dissipation shifts in QCM-D. In streptococci, the polyelectrolyte network surrounding a bacterium is formed by fibrillar surface appendages and spring constants derived from vibrational amplitudes decreased with increasing fibrillar density. In staphylococci, EPS constitutes the main network component, and larger amounts of EPS yielded higher spring constants. Spring constants increased with increasing ionic strength and strains with smaller electrophoretically derived bacterial cell surface softnesses possessed the highest spring constants

Centromere Tethering Confines Chromosome Domains

The organization of chromosomes into territories plays an important role in a wide range of cellular processes including gene expression, transcription, and DNA repair. Current understanding has largely excluded the spatio-temporal dynamic fluctuations of the chromatin polymer. We combine in vivo chromatin motion analysis with mathematical modeling to elucidate the physical properties that underlie the formation and fluctuations of territories. Chromosome motion varies in predicted ways along the length of the chromosome, dependent on tethering at the centromere. Detachment of a tether upon inactivation of the centromere results in increased spatial mobility. A confined bead-spring chain tethered at both ends provides a mechanism to generate observed variations in local mobility as a function of distance from the tether. These predictions are realized in experimentally determined higher effective spring constants closer to the centromere. The dynamic fluctuations and territorial organization of chromosomes are, in part, dictated by tethering at the centromere