Botanical ratchets.

Ratcheting surfaces are a common motif in nature and appear in plant awns and grasses. They are known to proffer selective advantages for seed dispersion and burial. In two simple model experiments, we show that these anisotropically toothed surfaces naturally serve as motion rectifiers and generically move in a unidirectional manner, when subjected to temporally and spatially symmetric excitations of various origins. Using a combination of theory and experiment, we show that a linear relationship between awn length and ratchet efficiency holds under biologically relevant conditions. Grass awns can thus efficiently transform non-equilibrium environmental stresses from such sources as humidity variations into useful work and directed motion using their length as a fluctuation amplifier, yielding a selective advantage to these organelles in many plant species.

Renormalization group in quantum mechanics at zero and finite temperature.

We apply the renormalization group formalism, to integrate quantum fluctuations of quantum mechanical systems at zero and finite temperature. At zero temperature a nonperturbative renormalization group equation allows to compute the ground state energy whereas at finite temperature a variational renormalization group equation is proposed to compute the free energy.