11 February 2000 Ž . Chemical Physics Letters 317 2000 647–651 www.elsevier.nlrlocatercplett Effect of medium viscosity in a closed unstirred Belousov–Zhabotinsky system Nadia Marchettini a, ) , Mauro Rustici b a Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi, UniÕersita di Siena, Pian dei Mantellini 44, 53100 Siena, Italy ` b Dipartimento di Chimica, UniÕersita di Sassari, Via Vienna 2, 07100 Sassari, Italy ` Received 29 September 1999; in final form 7 December 1999 Abstract Complex periodic and aperiodic behaviors are reported in an unstirred Belousov–Zhabotinsky oscillatory reaction containing different concentrations of polyethylene-glycol. A route from chaos following an inverse Ruelle–Takens–New- Ž . house RTN scenario is identified. Thus, hydrodynamic effects, being mainly convective, seem to be responsible for the observed RTN scenario. We demonstrate that the medium viscosity, namely the polyethylene-glycol concentration, is a bifurcation parameter for the sequence chaos quasi-periodicity period-1. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction An autocatalytic reaction performed in an un- stirred batch reactor can support a constant-velocity wavefront resulting from the coupling between diffu- wx sion and kinetics 1 . As a front propagates, concen- tration and thermal gradients are formed that alter the density of the solution, often causing convection w x 2,3 . Hydrodynamic movements can be promoted wx when spatial inhomogeneity arises 4. Hydrody- namic flows associated with chemical wave propaga- Ž . tion in a Belousov–Zhabotinsky BZ solution ex- cluding effects from evaporative cooling have been wx demonstrated 5 . Pojman and Epstein have classified the types of convection that can occur in a travelling ) Corresponding author. E-mail: nadia@envichem.chim.unisi.it wx front 1 . Most of the experimental and theoretical studies have been concerned with the influence of w x natural convection on travelling fronts 6–10 . The BZ reaction exhibits waves, and these have been w x extensively studied 11–13 . Recently, we have observed aperiodic oscillations during the chemical evolution of a closed unstirred w x cerium-catalyzed BZ system 17 . These aperiodic oscillations are an example of transient chaos be- cause they are sensitive to the initial conditions w x which is the major distinctive feature of chaos 14 . The chaotic regime is bounded by two periodic zones. The onset of chaos spontaneously starts by a Ž . w x Ruelle–Takens–Newhouse RTN scenario 15,17 as soon as convection motion couples to diffusion and local kinetics. The chaotic motion continues for w x about 2 h and ends by an inverse RTN scenario 16 . Ž . In contrast to the first transition RTN scenario , the 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 01411-6