No abstract

Observations of steady nonaxisymmetric chemical wave fronts are reported for upward propagation in iodatearsenous acid solutions within vertical capillary tubes. These observations confirm a recent prediction of hydrodynamic stability theory that the onset of convection in such fronts should be nonaxisymmetric. The nonaxisymmetric waveform reflects the presence of a single convective roll in the vicinity of the moving front. IntroductionUnderstanding convective effects in chemical waves represents an interesting and important challenge. Chemical waves create temperature and concentration gradients which can lead to mass density gradients. In the presence of gravity, these density gradients can destabilize planar reaction-diffusion waves, leading to fluid convection and the development of curved fronts. This convection is the source of differences between ascending and descending front propagation speeds in the iron(I1)-nitric acid reaction,1.2 the chloritethiosulfate reaction: and the iodatearsenous acid r e a c t i~n~.~ in vertical capillary tubes. Measurements of front speed provide simple tests of proposed reaction-diffusion mechanisms;s+6 hence, it is important to know when convection contributes to the speed of propagation. Furthermore, it is of interest to understand and predict the effects of convection on the waveform of the propagating front.Previous experiments on iodatearsenous acid mixture3g4 revealed steady, curved fronts and an increase in propagation speed with increasing tube diameter for ascending waves. The iodate-arsenous acid reaction produces a reacted solution which is less dense than the unreacted solution, so that only upward propagation (with the lighter fluid below) is potentially unstable under the action of gravity. Indeed, descending fronts initiated at the top of the tube remain flat and propagate at a speed independent of the tube diameter. For a tube of diameter 0.94 mm, ascending fronts were also flat and had the same speed as descending fronts, indicating the absence of convection. For diameters of 1.8 mm and above, axisymmetric curved fronts were observed with an approximately parabolic profile and the highest point in the center of the cylindrical tube. The curvature of the ascending fronts increased with increasing tube diameter as did the speed. This curvature was attributed to convection driven by the buoyancy of the reacted fluid, with fluid moving up near the center of the cylinder and down near the cylinder walls.

We studied drops of dodecyl acrylate in poly(dodecyl acrylate) (molecular weight of 25,000) in a spinning drop tensiometer to determine whether an effective interfacial tension (EIT) existed between these two miscible fluids. We found convincing evidence. We estimated the mechanical relaxation time from an immiscible analogue (1-propanol and poly(dodecyl acrylate)) and showed that the dodecyl acrylate drops maintained quasi-steady diameters long after this relaxation period. Drops continuously grew in length and became more diffuse, but the width of the transition zone did not grow with t(1/2) as expected from Fick's law although this system had been shown to follow Fick's law in a static configuration (Antrim, D.; Bunton, P.; Lewis, L. L.; Zoltowski, B. D.; Pojman, J. A. J. Phys. Chem. B 2005, 109, 11842-11849). The EIT was determined from Vonnegut's equation, EIT = (Deltarho)omega(2)r(3)/4; both the inner and outer diameters were measured, yielding values of 0.002 and 0.02 mN m(-1), respectively. The EIT was found to be independent of the rotation rate above 6000 rpm and independent of the initial drop volume. The EIT was found to decrease with temperature and increase with the difference in concentration between the monomer drop and polymer-monomer fluid. The square gradient parameter, k, was determined from EIT = k(Deltac(2)/delta), where Deltac is the difference in mole fraction and delta is the width of the transition zone. The square gradient parameter was on the order of 10(-9) N. The square gradient parameter was found to decrease with temperature, to be independent of concentration, and to increase with the molecular weight of the polymer.

Front dynamics in the frontal polymerization of two multifunctional acrylate monomers, 1,6-hexanediol diacrylate (HDDA) and trimethylolpropane ethoxylate triacrylate (TMPTA), with Lupersol 231 [1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane] as the initiator, are studied. In most frontal polymerization systems, the dynamics are associated with a planar front propagating through the sample. However, in some cases, front behavior can be altered: the front becomes nonplanar characterized by complex patterns like spin modes and pulsations. To determine how these periodic and aperiodic modes arise, reactant solutions consisting of HDDA diluted with diethyl phthalate (DEP) and TMPTA diluted with dimethyl sulfoxide (DMSO) were used in the study. In the study we reveal frontal behavior characteristic of period-doubling behavior, a doubling of spin heads that degenerate into an apparently chaotic mode. Also, a pulsating symmetric mode has been observed. These observations have a striking similarity to observations made in studies of self-propagating high-temperature synthesis (SHS) in which the addition of an inert diluent afforded a rich variety of dynamical behavior. The degree of cross-linking has also been found to be a bifurcation parameter. The energy of activation of multifunctional acrylate polymerization is a strong function of the degree of polymerization. By adding a monoacrylate (benzyl acrylate: BzAc), such that the front temperature was invariant, we observed a period-doubling bifurcation sequence through changes in the energy of activation, which has not been previously reported. (c) 1999 American Institute of Physics.