Poly NIPAM Microgels with Different Cross Linker Densities Scaling Behavior of the Network Fluctuations in the Vicinity of the Volume Phase Transition

Thermoresponsive microgel particles made of the monomer N isopropylacrylamide NIPAM and the cross linker molecule N,N methylenebisacrylamide BIS were synthesized using three different cross linker molar ratios. The volume phase transition behavior of these colloids was investigated by means of dynamic light scattering DLS and small angle neutron scattering SANS covering the different length scales of interest. Both methods provide the temperature of the volume phase transition in good agreement. The volume change as followed by DLS is described using the Flory Rehner theory, leading to the determination of the spinodal temperature. Furthermore, the network correlation length amp; 958;, which is available from appropriate fits of the measured SANS profiles, was used to study the critical behavior in terms of scaling laws. The results from DLS and SANS show a strong cross linker density dependenc

Stroboscopic Small Angle Neutron Scattering Investigations of Microsecond Dynamics in Magnetic Nanomaterials

Time resolved Small Angle Neutron Scattering SANS techniques have recently been developed that allow ordering and relaxation processes of magnetic moments in nanoparticles to be monitored. In stroboscopic experiments, time frame data acquisition has been synchronized with a periodic external magnetic field. Slow relaxation of magnetic particle moments onto equilibrium has been studied in periods of the order of 30 s after switch off a static field. By applying a sine wave modulated magnetic field at frequencies above 50 Hz, the time resolved SANS response to a forced oscillation could be analyzed. When a continuous neutron flux was used in conventional SANS, the shortest accessible time range was limited to about 3 ms resulting from the wavelength spread. A breakthrough of time resolution into the micro second range was achieved with the pulsed frame overlap TISANE technique, which allows us to exploit a dynamical range similar to that of X ray photon correlation spectroscopy. Here we present a combination of these stroboscopic neutron techniques on surfactant stabilized ferrofluids with nearly monodisperse Cobalt and Fe3O4 nanoparticles. Results are compared to a solid CuCo alloy with superparamagnetic nanosized Cobalt precipitates. The SANS scattering response was measured stroboscopically in an oscillating applied magnetic field at frequencies up to 2800 Hz. As long as the magnetic moments followed the applied field, the 2D scattering patterns alternated between fully isotropic and strongly anisotropic. The analysis of time dependent SANS data as a function of frequency, field and temperature allowed us i to proof the validity of the Langevin statistics describing the particle moment orientation, ii to extract the effect of field induced interparticle correlations, iii to monitor the slowing down of the dynamics of moment rotation with decreasing temperature, iv to study the effect of freezing of the solvent on the dynamics of the particle moments, and v to decide between the possible relaxation mechanisms N el and Brownia

Coping with crowds : density-dependent disease resistance in desert locusts.

Parasite transmission generally exhibits some form of positive density dependence. Thus, as population density increases, so too does the per capita risk of becoming infected. Under such circumstances, natural selection should favor individuals that use cues associated with population density to determine the optimal allocation of resources to disease resistance mechanisms. As a consequence, individuals experiencing crowded conditions are predicted to be more resistant to parasites and pathogens than those experiencing low-density conditions. This phenomenon (termed "density-dependent prophylaxis") [Wilson, K. & Reeson, A. F. (1998) Ecol. Entomol. 23, 100-101] is predicted to be particularly prevalent in outbreak pest species and in species exhibiting density-dependent phase polyphenism, such as the desert locust, Schistocerca gregaria. Here we show that, as predicted, desert locusts reared under crowded conditions are significantly more resistant than solitary locusts to the entomopathogenic fungus, Metarhizium anisopliae var. acridum, a key natural disease of acridids and an important agent in locust and grasshopper biocontrol. Moreover, enhanced pathogen resistance in crowded locusts is associated with elevated antimicrobial activity, but not with any difference in thermal preferences or behavioral fever response. These results have implications for understanding the development and biocontrol of locust plagues