Transverse polarizability of an aligned assembly of charged rods

The low-field transverse polarizability of the counterions condensed on an isolated charged rod is small. We show that it can be much larger if the rod is a member of an assembly of aligned rods. The polarization free energy of the assembly of rods in a transverse field is then similar to its polarization free energy in a field parallel to the rods. The polarization free energy of the assembly in a transverse field becomes lower than in a parallel field if the extent of the assembly (as measured, for example, by the diameter of a cylindrical assembly) is larger than the length of the individual rods. We suggest that this model may provide a reasonable explanation for the occurrence of “anomalous” birefringence in systems of interacting charged rodlike particles.     

Three-dimensional molecular mapping in a microfluidic mixing device using fluorescence lifetime imaging

Fluorescence lifetime imaging (FLIM) is used to quantitatively map the concentration of a small molecule in three dimensions in a microfluidic mixing device. The resulting experimental data are compared with computational fluid-dynamics (CFD) simulations. A line-scanning semiconfocal FLIM microscope allows the full mixing profile to be imaged in a single scan with submicrometer resolution over an arbitrary channel length from the point of confluence. Following experimental and CFD optimization, mixing times down to 1.3+/-0.4 ms were achieved with the single-layer microfluidic device.     

Improved sectioning in a slit scanning confocal microscope

We describe a simple implementation of a slit scanning confocal microscope to obtain an axial resolution better than that of a point-scanning confocal microscope. Under slit illumination, images of a fluorescent object are captured using an array detector instead of a line detector so that out-of-focus light is recorded and then subtracted from the adjacent images. Axial resolution after background subtraction is 2.2 times better than the slit confocal resolution, and out-of-focus image suppression is calculated to attenuate with defocus faster by 1 order of magnitude than in the point confocal case.     

Potentiometric titrations with a mercury-mercury sulphide electrode

Platinum foil coated with a film of mercury and mercury sulphide is recommended as electrode for potentiometric titrations with thioacetamide.     

Fluoride binding to humic acid

Fluoride (F^–) binding to humic acid has been measured as a function of pH (5–6.6). The pH dependent binding is attributed to the anion being trapped within the large structure (territorial bound) but is not bound to a particular functional group (site bound). Studying fluoride binding provides insight to cation, anion and neutral species interactions with humic acid.