Drying and cracking mechanisms in a starch slurry

Starch-water slurries are commonly used to study fracture dynamics. Drying starch-cakes benefit from being simple, economical, and reproducible systems, and have been used to model desiccation fracture in soils, thin film fracture in paint, and columnar joints in lava. In this paper, the physical properties of starch-water mixtures are studied, and used to interpret and develop a multiphase transport model of drying. Starch-cakes are observed to have a nonlinear elastic modulus, and a desiccation strain that is comparable to that generated by their maximum achievable capillary pressure. It is shown that a large material porosity is divided between pore spaces between starch grains, and pores within starch grains. This division of pore space leads to two distinct drying regimes, controlled by liquid and vapor transport of water, respectively. The relatively unique ability for drying starch to generate columnar fracture patterns is shown to be linked to the unusually strong separation of these two transport mechanisms.Comment: 9 pages, 8 figures [revised in response to reviewer comments

Drying and cracking mechanisms in a starch slurry

Starch-water slurries are commonly used to study fracture dynamics. Drying starch-cakes benefit from being simple, economical, and reproducible systems, and have been used to model desiccation fracture in soils, thin film fracture in paint, and columnar joints in lava. In this paper, the physical properties of starch-water mixtures are studied, and used to interpret and develop a multiphase transport model of drying. Starch-cakes are observed to have a nonlinear elastic modulus, and a desiccation strain that is comparable to that generated by their maximum achievable capillary pressure. It is shown that a large material porosity is divided between pore spaces between starch grains, and pores within starch grains. This division of pore space leads to two distinct drying regimes, controlled by liquid and vapor transport of water, respectively. The relatively unique ability for drying starch to generate columnar fracture patterns is shown to be linked to the unusually strong separation of these two transport mechanisms.Comment: 9 pages, 8 figures [revised in response to reviewer comments

Fundamental investigation of the drying of solid suspensions

In this work, a comprehensive series of experiments is conducted to investigate the drying behaviour of micro- and nano-sized particle dispersions. To this end, an acoustic levitator was used to study the drying kinetics of single droplets. The temporal evolution of the actual droplets was recorded using a CMOS camera and the solid grains produced at the end of drying were investigated by SEM imaging. At the end of drying, the grains show different morphologies as a function of the particle size, concentration and initial droplet volume. We combine these experimental data to show the drying behaviour is dependent on all the parameters and that the data all collapses when plotted against Péclet number. This resulted in a novel characteristic diagram which allows one to predict the shape of the dried colloidal droplet based on Pé. Our results extend the fundamental understanding of the mechanisms controlling drying of droplet suspensions

Mechanochemical feedback regulates the dynamics of the PAR system in C. elegans zygotes

The interplay between regulatory biochemistry and cell mechanics is critical for a broad range of morphogenetic changes. Cell mechanics can induce transport via growth and flow-fields, which in turn affect concentration-fields of regulators. Such systems exhibit an intrinsic feedback-architecture between regulators of cell mechanics and mechanical deformation. While we anticipate that this feedback between biochemistry and cell mechanics is widespread in Morphogenesis, there are few examples that are studied with respect to their potential for generating spatiotemporal patterns. Here we establish at a quantitative level that PAR polarization of C. elegans zygotes represents a coupled mechanochemical system. Using Fluorescence Recovery After Photobleaching (FRAP) and RNA interference (RNAi), we first demonstrate that the biochemistry in form of the PAR domains feeds back on the mechanics by establishing and maintaining a non-muscle myosin II (nmy-2) gradient. Additionally, we characterize the effect of the polarity cue associated with the centrosome of the male pronucleus on the local myosin concentration at the posterior pole. We show that it induces a reduction in myosin concentration and thereby triggers the onset of cortical flows. Furthermore we measure the spatiotemporal profile of the anterior and posterior PAR concentration, the myosin II concentration and the induced flow-field. Finally, we capture the feedback-architecture of the coupled actomyosin – PAR system in a quantitative model, based on coupling a thin film active fluid description of cortical mechanics [1] to a reaction-diffusion PAR patterning system [2]. We show that this mathematical model can quantitatively recapitulate the spatiotemporal profile of PAR polarity establishment. Furthermore, we demonstrate that the model predicts the existence of a threshold in cortical flow velocity, which separates the nonpolarizing and the polarizing regime and confirm the existence of this threshold velocity in the living C. elegans zygote

Effects of dried distillers grains with solubles on sow carcass fat quality

A pilot experiment was conducted to determine the effects of feeding nonpregnant (open) sows a diet containing 50% dried distillers grains with solubles (DDGS) on growth and carcass fat quality. A total of 8 open sows were allotted to 1 of 2 diets by parity and BW. One diet was a standard corn-soybean meal-based gestation diet; the second diet was a corn-soybean meal-based diet that contained 50% DDGS. All sows were fed 5 lb/d of feed in a single feeding for 92 d. All sows were harvested on d 92 at the Kansas State University Meat Laboratory for determination of carcass fat quality. As expected, no differences in BW or backfat change were found (P \u3e 0.62) for the feeding period. Additionally, no differences (P \u3e 0.23) in lipid oxidation as measured by 2-thiobarbituric acid reactive substances (TBARS) assay were reported either initially or after 5 d of retail display for sows fed 50% DDGS compared with controls. Lipid oxidation increased (P \u3c 0.003) as measured by TBARS assay for both treatments from d 1 to 5 as expected. Jowl fatty acid analysis revealed an increase in linoleic acid (P \u3c 0.01), total polyunsaturated fatty acids (P \u3c 0.01), and the ratio of polyunsaturated fatty acids to saturated fatty acids (P \u3c 0.03). Also, there was a trend for increased jowl iodine value (P \u3c 0.08) for sows fed 50% DDGS compared with the controls. In summary, feeding 50% DDGS to open sows for 92 d did not significantly affect BW, backfat, and lipid oxidation compared with controls. However, feeding 50% DDGS increased the concentration of linoleic acid and total polyunsaturated fatty acids and tended to increase jowl iodine value compared with controls.; Swine Day, 2008, Kansas State University, Manhattan, KS, 200

Dynamic nuclear polarization and spin-diffusion in non-conducting solids

There has been much renewed interest in dynamic nuclear polarization (DNP), particularly in the context of solid state biomolecular NMR and more recently dissolution DNP techniques for liquids. This paper reviews the role of spin diffusion in polarizing nuclear spins and discusses the role of the spin diffusion barrier, before going on to discuss some recent results.Comment: submitted to Applied Magnetic Resonance. The article should appear in a special issue that is being published in connection with the DNP Symposium help in Nottingham in August 200

The Extracytoplasmic Domain of the Mycobacterium tuberculosis Ser/Thr Kinase PknB Binds Specific Muropeptides and Is Required for PknB Localization

The Mycobacterium tuberculosis Ser/Thr kinase PknB has been implicated in the regulation of cell growth and morphology in this organism. The extracytoplasmic domain of this membrane protein comprises four penicillin binding protein and Ser/Thr kinase associated (PASTA) domains, which are predicted to bind stem peptides of peptidoglycan. Using a comprehensive library of synthetic muropeptides, we demonstrate that the extracytoplasmic domain of PknB binds muropeptides in a manner dependent on the presence of specific amino acids at the second and third positions of the stem peptide, and on the presence of the sugar moiety N-acetylmuramic acid linked to the peptide. We further show that PknB localizes strongly to the mid-cell and also to the cell poles, and that the extracytoplasmic domain is required for PknB localization. In contrast to strong growth stimulation by conditioned medium, we observe no growth stimulation of M. tuberculosis by a synthetic muropeptide with high affinity for the PknB PASTAs. We do find a moderate effect of a high affinity peptide on resuscitation of dormant cells. While the PASTA domains of PknB may play a role in stimulating growth by binding exogenous peptidoglycan fragments, our data indicate that a major function of these domains is for proper PknB localization, likely through binding of peptidoglycan fragments produced locally at the mid-cell and the cell poles. These data suggest a model in which PknB is targeted to the sites of peptidoglycan turnover to regulate cell growth and cell division

Membrane Invaginations Reveal Cortical Sites that Pull on Mitotic Spindles in One-Cell C. elegans Embryos

Asymmetric positioning of the mitotic spindle in C. elegans embryos is mediated by force-generating complexes that are anchored at the plasma membrane and that pull on microtubules growing out from the spindle poles. Although asymmetric distribution of the force generators is thought to underlie asymmetric positioning of the spindle, the number and location of the force generators has not been well defined. In particular, it has not been possible to visualize individual force generating events at the cortex. We discovered that perturbation of the acto-myosin cortex leads to the formation of long membrane invaginations that are pulled from the plasma membrane toward the spindle poles. Several lines of evidence show that the invaginations, which also occur in unperturbed embryos though at lower frequency, are pulled by the same force generators responsible for spindle positioning. Thus, the invaginations serve as a tool to localize the sites of force generation at the cortex and allow us to estimate a lower limit on the number of cortical force generators within the cell

Phosphorylation Provides a Negative Mode of Regulation for the Yeast Rab GTPase Sec4p

The Rab family of Ras-related GTPases are part of a complex signaling circuitry in eukaryotic cells, yet we understand little about the mechanisms that underlie Rab protein participation in such signal transduction networks, or how these networks are integrated at the physiological level. Reversible protein phosphorylation is widely used by cells as a signaling mechanism. Several phospho-Rabs have been identified, however the functional consequences of the modification appear to be diverse and need to be evaluated on an individual basis. In this study we demonstrate a role for phosphorylation as a negative regulatory event for the action of the yeast Rab GTPase Sec4p in regulating polarized growth. Our data suggest that the phosphorylation of the Rab Sec4p prevents interactions with its effector, the exocyst component Sec15p, and that the inhibition may be relieved by a PP2A phosphatase complex containing the regulatory subunit Cdc55p