Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose

Stable colloidal suspensions of cellulose microcrystallites may be prepared from filter paper by sulfuric acid hydrolysis. Above a critical concentration, the suspensions form a chiral nematic ordered phase, or colloid crystal. The preparation conditions govern the properties of the individual cellulose microcrystallites, and hence the liquid crystalline phase separation of the cellulose suspensions. The particle properties and the phase separation of the suspensions were strongly dependent on the hydrolysis temperature and time, and on the intensity of the ultrasonic irradiation used to disperse the particles. The particle size of the microcrystallites was characterized with transmission electron microscopy and photon correlation spectroscopy. The surface charge was determined by conductometric titration. It was possible to fractionate the microcrystallites by size using the partitioning between isotropic and liquid crystalline phases; the longer microcrystallites migrate to the liquid crystalline phase     

Atomic force microscopy and transmission electron microscopy of cellulose from Micrasterias denticulata; evidence for a chiral helical microfibril twist

Atomic force microscopy (AFM), tapping mode atomic force microscopy (TM-AFM) and transmission electron microscopy (TEM) have been used to image the cell wall, ultrathin sections of whole cells and cellulose microfibrils prepared from the green alga Micrasterias denticulata. Measurements of the microfibril dimensions are in agreement with earlier observations carried out by electron microscopy. Images at the molecular level of the surface of the microfibrils were obtained with AFM and show regular periodicities along the microfibril axis that correspond to the fibre and glucose repeat distances of cellulose. Twisted regions visible at intervals along the microfibrils dried down onto substrates were noted to be right-handed in over 100 observations by TEM, AFM and TM-AFM. This revised version was published online in August 2006 with corrections to the Cover Date.     

POPULATION‐LEVEL ANALYSIS AND VALIDATION OF AN INDIVIDUAL‐BASED CUTTHROAT TROUT MODEL

ABSTRACT. An individual‐based model of stream trout is analyzed by testing its ability to reproduce patterns of population‐level behavior observed in real trout: (1) “self‐thinning,” a negative power relation between weight and abundance; (2) a “critical period” of density‐dependent mortality in young‐of‐the‐year; (3) high and age‐specific inter‐annual variability in abundance; (4) density dependence in growth; and (5) fewer large trout when pool habitat is eliminated. The trout model successfully reproduced these patterns and was useful for evaluating their theoretical basis. The model analyses produced new explanations for some field observations and indicated that some patterns are less general than field studies indicate. The model did not reproduce field‐observed patterns of population variability by age class, discrepancies potentially explained by site differences, predation mortality being more stochastic than the model assumes, or uncertainty in the field study's age estimates.