Dennis Tierney

Nonlinearity in the rotational dynamics of Haidinger's brushes

Haidingers brushes are an entoptic effect of the human visual system that enables us to detect polarized light. However, individual perceptions of Haidingers brushes can vary significantly. We find that the birefringence of the cornea influences the rotational motion and the contrast of Haidingers brushes and may offer an explanation for individual differences. We have devised an experimental setup to simulate various phase shifts of the cornea and found a switching effect in the rotational dynamics of Haidingers brushes. In addition, age related macular degeneration reduces the polarization effect of the macula and thus also leads to changes in the brush pattern.

How Rosalind Franklin Discovered the Helical Structure of DNA: Experiments in Diffraction

Rosalind Franklin, a chemical physicist (1920–1958), used x‐ray diffraction to determine the structure of DNA. What exactly could she read out from her x‐ray pattern, shown in Fig. 1?1 In lecture notes dated November 1951, R. Franklin wrote the following: “The results suggest a helical structure (which must be very closely packed) containing 2, 3 or 4 co‐axial nucleic acid chains per helical unit, and having the phosphate groups near the outside.”2 This was 16 months before J. D. Watson and F. Crick published their description of DNA, which was based on R. Franklins x‐ray photos. How they gained access to her x‐ray photos is a fascinating tale of clashing personalities and male chauvinism.2,3

Optically driven bacterial screw of Archimedes

The linear momentum transfer from photons to asymmetrically shaped structures generates an optomechanical rotation, but these micron-sized structures require costly, high-precision fabrication. Nature, however, provides a great diversity of engineered forms and dead, but rigid, asymmetrically shaped bacteria can provide a low cost alternative. In this work, we show that helical bacteria rotate very quickly in optical tweezers. Their rotation is fast enough to create a strong whirl, such that they may act as micropumps in thin capillaries.

Holograms: made from scratch

Scratch holograms can be described as the limiting case of geometrical optics, where k→ ∞ , in which photons behave like grains of sand. We will show that scratch holograms can in principle be produced and read out by white light interference.

Bacteria as Micro Mechanical Augers

Optical tweezers can drive the rotation of micro sized, asymmetrically shaped structures. Since evolution provides a great diversity of engineered forms, we show that Rhodospirillum rubrum can be used as light driven micro mixers.