Carl Franck

Interfacial tensions of phase-separated polymer solutions

Measurements of the capillary length in phase‐separated solutions of polymethylmethacrylate in 3‐octanone (PMMA/3‐OCT) for each of five different molecular weights of PMMA, and in phase‐separated solutions of polystyrene in methylcyclohexane (PS/MCH) for a single molecular weight of polymer, are reported. Measurements on PMMA/3‐OCT were by the sessile‐drop method; those on PS/MCH were by both the sessile‐drop and capillary‐rise methods. Interfacial tensions are estimated from estimated density differences and the measured capillary lengths. A theoretically predicted scaling law for the dependence of capillary length on polymer molecular weight and temperature is tested and at least semiquantitatively verified. As by‐products of the sample preparation, the molecular‐weight dependences of the critical composition and critical solution temperature of PMMA/3‐OCT were also determined. The volume fraction of polymer at the critical point, φc, is found to be proportional to the −0.37±0.01 power of the molecular ...

A positron-sensitive photon detector for the UV or X-ray range

Abstract This paper describes the conversion of a light sensitive self-scanning silicon photodiode array into a soft X-ray detector. We combine a photodiode array, a UHV compatible soft X-ray sensitive phosphor and read out electronics. The detector has been tested in the soft X-ray and UV regions. The results indicate a high quantum efficiency in the soft X-ray region.

A new toroidal grating spectrometer for the soft x-ray region

Abstract We have developed toroidal grating instrument using holography aberration corrected gratings to give a flat field focus and cover the wavelength region 16–625 A. The spectrometer uses four interchangeable gratings as analysers and a self scanning silicon array as detector. The sample chamber is a bakeable UHV system with LEED and Auger surface analysis equipment to characterize the sample surface.

Live cell flattening — traditional and novel approaches

Eukaryotic cell flattening is valuable for improving microscopic observations, ranging from bright field (BF) to total internal reflection fluorescence (TIRF) microscopy. Fundamental processes, such as mitosis and in vivo actin polymerization, have been investigated using these techniques. Here, we review the well known agar overlayer protocol and the oil overlay method. In addition, we present more elaborate microfluidics-based techniques that provide us with a greater level of control. We demonstrate these techniques on the social amoebae Dictyostelium discoideum, comparing the advantages and disadvantages of each method. PACS Codes: 87.64.-t, 47.61.-k, 87.80.Ek

New soft x‐ray emission spectrograph

We have built a new soft x‐ray emission spectrograph covering the photon energy range 20–800 eV. It incorporates toroidal holographic grazing incidence diffraction gratings and a position‐sensitive photodiode array as a detector. The detector electronics are remote from the array which is under vacuum at nitrogen temperature, and features a double‐correlated sampling scheme. The sample is excited with a Pierce‐type electron gun using a quadrupole focusing lens. The performance of the instrument is described.

Spontaneous emergence of large-scale cell cycle synchronization in amoeba colonies.

Unicellular eukaryotic amoebae Dictyostelium discoideum are generally believed to grow in their vegetative state as single cells until starvation, when their collective aspect emerges and they differentiate to form a multicellular slime mold. While major efforts continue to be aimed at their starvation-induced social aspect, our understanding of population dynamics and cell cycle in the vegetative growth phase has remained incomplete. Here we show that cell populations grown on a substrate spontaneously synchronize their cell cycles within several hours. These collective population-wide cell cycle oscillations span millimeter length scales and can be completely suppressed by washing away putative cell-secreted signals, implying signaling by means of a diffusible growth factor or mitogen. These observations give strong evidence for collective proliferation behavior in the vegetative state.

Inelastic x‐ray scattering with fluorescence coincidence detection using a pulsed synchrotron source

We describe a system used to measure the spectrum of x rays scattered inelastically from K‐shell electrons using a coincidence technique. The source of the x rays was a pulsed synchrotron source. We describe fast–slow coincidence logic adapted for use with a pulsed source of fluctuating intensity, describe in detail the background subtraction and statistical noise in the measurement, and present the results of measurements made with the system. The precautions used to avoid detector‐to‐detector scattering are also described. The combination of high source intensity and coarse time resolution (due to the pulsed nature of the source) put us in the regime for much of the scattered x‐ray spectrum, in which the number of accidental coincidences greatly exceeded the number of true signal counts. We show that in this case significant reductions in statistical uncertainty may not be achieved by further increases in source intensity.

Classical scaling functions for moments of the order parameter profile in critical adsorption of a binary liquid mixture

Because knowledge of the moments of the order parameter profile is required to understand reflected light studies of critical adsorption of a liquid mixture, the scaling functions in classical Landau theory for the two lowest order moments have been calculated. A contact substrate‐liquid interaction of strength h1 is presumed. The first moment is found to saturate at large scaled h1. Finally, the connection with a recent experiment is discussed.

High fidelity information processing in folic acid chemotaxis of Dictyostelium amoebae

Living cells depend upon the detection of chemical signals for their existence. Eukaryotic cells can sense a concentration difference as low as a few per cent across their bodies. This process was previously suggested to be limited by the receptor–ligand binding fluctuations. Here, we first determine the chemotaxis response of Dictyostelium cells to static folic acid gradients and show that they can significantly exceed this sensitivity, responding to gradients as shallow as 0.2% across the cell body. Second, using a previously developed information theory framework, we compare the total information gained about the gradient (based on the cell response) to its upper limit: the information gained at the receptor–ligand binding step. We find that the model originally applied to cAMP sensing fails as demonstrated by the violation of the data processing inequality, i.e. the total information exceeds the information at the receptor–ligand binding step. We propose an extended model with multiple known receptor types and with cells allowed to perform several independent measurements of receptor occupancy. This does not violate the data processing inequality and implies the receptor–ligand binding noise dominates both for low- and high-chemoattractant concentrations. We also speculate that the interplay between exploration and exploitation is used as a strategy for accurate sensing of otherwise unmeasurable levels of a chemoattractant.

NONEQUILIBRIUM INTERFACIAL TENSION MEASUREMENTS ON POLYMER SOLUTIONS

The observation of critical phenomena in polymer solutions is hindered by the slow approach of such systems to equilibrium. We have addressed this problem with a variation of the sessile drop technique that allows equilibrium interfacial properties to be obtained from systems far from equilibrium. Following a temperature jump, the interfacial region of a drop responds rapidly compared with the bulk phases, and, despite significant flux of polymer and solvent across it, the interface can reveal information about the equilibrium state well before bulk equilibration is complete. Interfacial data from these systems show good agreement with data from equilibrated samples. The kinetics of solvent and polymer transport across the interface following a temperature perturbation are also examined.