Rogério Martins

The (Not So Simple!) Chain Fountain

ABSTRACT Given a sufficiently long bead chain in a cup, if we pull the end of the chain over the rim of the cup, the chain tends to continuously “flow” out of the cup, under gravity, in a common siphon process. Surprisingly enough, under certain conditions, the chain forms a fountain in the air! This became known as the Mould effect, after Steve Mould who discovered this phenomenon and made this experiment famous on YouTube [Mould 13] in a video that went viral. The reason for the emergence of this fountain remains unclear. This effect was shown [Biggins and Warner 14] to be due to an anomalous reaction force from the top of the pile of beads; a possible origin for this force was proposed in the same paper. Here, we describe some experiments that contribute toward the clarification of the origin of this force, and show that the explanation goes far beyond the one proposed in [Biggins and Warner 14].

On the Effect of a Rotating Magnetic Field or Sample on the Bend Freedericksz Critical Field in Nematic Slabs

A numerical study of the effect of a continuously rotating field or sample on the magnetic Freedericksz transition in homeotropic nematics slabs is presented. Homeotropic boundary conditions with strong anchoring are considered. The magnetic field is applied parallel to the plates, i.e., in the bend Freedericksz geometry. The behaviour of a low molecular weight liquid crystal (5CB) and of a high molecular weight polymer liquid crystal (PBLG) is compared using the Leslie-Ericksen theory of nematodynamics. The effect of the sample thickness, of the magnetic field strength and of the spinning frequency on the response of both materials is studied. Our results show that, in both cases and for both materials, the value of the Freedericksz critical field H c (ν) increases with the spinning frequency ν. The transition H c (ν) → H c (0) is found to be continuous in the rotating sample case and to be discontinuous in the rotating field case. The plot of the reduced critical field vs. the reduced spinning frequency gives universal curves for both materials in both cases.