Annette F. Taylor

Dynamical Quorum Sensing and Synchronization in Large Populations of Chemical Oscillators

Populations of certain unicellular organisms, such as suspensions of yeast in nutrient solutions, undergo transitions to coordinated activity with increasing cell density. The collective behavior is believed to arise through communication by chemical signaling via the extracellular solution. We studied large, heterogeneous populations of discrete chemical oscillators (∼100,000) with well-defined kinetics to characterize two different types of density-dependent transitions to synchronized oscillatory behavior. For different chemical exchange rates between the oscillators and the surrounding solution, increasing oscillator density led to (i) the gradual synchronization of oscillatory activity, or (ii) the sudden “switching on” of synchronized oscillatory activity. We analyze the roles of oscillator density and exchange rate of signaling species in these transitions with a mathematical model of the interacting chemical oscillators.

Base-Catalyzed Feedback in the Urea−Urease Reaction

The bell-shaped rate-pH curve coupled to production of base in the urea-urease reaction was utilized to give feedback-driven behavior: an acid-to-base pH clock (a kinetic switch), bistability and hysteresis between an acid/base state when the initial pH was adjusted by a strong acid, and aperiodic pH oscillations when the initial pH was adjusted by a weak acid in an open reactor. A simple model of the reaction reproduced most of the experimental results and provided insight into the role of self-buffering in the dynamics. This reaction suggests new possibilities in the development of biocompatible feedback to couple to pH-sensitive processes for bioinspired applications in medicine, engineering, or materials science.

Phase Clusters in Large Populations of Chemical Oscillators

and suggest that many types of systems, including coupledunicellular organisms, may be capable of both types ofsynchronization transitions.The classical Kuramoto synchronization transition occurssmoothly above the critical coupling strength, and thefrequency and phase of the oscillators become increasinglyaligned with increasing coupling strength.

pH Wave-Front Propagation in the Urea-Urease Reaction

The urease-catalyzed hydrolysis of urea displays feedback that results in a switch from acid (pH ~3) to base (pH ~9) after a controllable period of time (from 10 to >5000 s). Here we show that the spatially distributed reaction can support pH wave fronts propagating with a speed of the order of 0.1-1 mm min(-1). The experimental results were reproduced qualitatively in reaction-diffusion simulations including a Michaelis-Menten expression for the urease reaction with a bell-shaped rate-pH dependence. However, this model fails to predict that at lower enzyme concentrations, the unstirred reaction does not always support fronts when the well-stirred reaction still rapidly switches to high pH.

Wave initiation in the ferroin-catalysed Belousov-Zhabotinsky reaction with visible light

The initiation of chemical reaction–diffusion waves by visible light of wavelength λ=633 nm from a 20 mW He–Ne laser in the ferroin-catalysed BZ reaction on a polysulfone membrane is repor ted. With low loading of the catalyst on the membrane, oxidation waves can be initiated from the resting steady state and in the recovering tail of a wave. With high loading, waves can only be initiated in the ‘vulnerable’ region behind an existing wavefront. The mechanism of this initiation is discussed in terms of the photoreduction of the metal–ligand catalyst and expressed in terms of a modified Oregonator model. These new observations are in contrast to the inhibitory effect of visible light in the light-sensitive Ru-catalysed BZ system.

Effect of oxygen on wave propagation in the ferroin-catalysed Belousov–Zhabotinsky reaction

An experimental investigation of the propagation of reaction-diffusion waves in thin layers of solution for the ferroin-catalysed Belousov–Zhabotinsky reaction is reported. It is shown that for layers of 1.5 mm or greater depth, there are no significant effects of oxygen on the wave speed, but that for more shallow layers the waves are sensitive to the presence of O2 in the gas phase above the solution and also to the concentration of malonic acid in the solution. Waves in solutions under N2 or solutions covered with a perspex lid show effectively no depth effects. Dispersion relationships, the dependence of the wave speed on wavelength, are determined and tested against a recently proposed ‘universal form’. Direct imaging of the waves shows that in the presence of O2, there is no wave propagation in the top 0.4 mm of solution, independent of the solution depth.

Role of differential transport in an oscillatory enzyme reaction

As a result of the bell-shaped pH-rate characteristic of enzymatic processes, feedback may arise in enzyme reactions having non-neutral products. This special type of product activation has been shown to lead to self-sustained pH oscillations in an enzyme-loaded membrane. We investigate the possibility of oscillations in a model of the urea-urease reaction, prompted by the recent experimental discovery of feedback in this reaction. An open system is considered in which acid and urea are transported to a cell containing the enzyme. Using linear stability analysis we determine the range of transport coefficients limit cycles may exist for and show that differential transport is required for oscillations in a class of compartmentalized enzyme processes similar to the urea-urease system. We demonstrate that although the transport rate of acid (k(H)) must be greater than that of urea (k(S)) for oscillations in a urease-loaded membrane, bistability is possible for k(S) ≥ k(H).

Magnetic resonance imaging of chemical waves in porous media

Magnetic resonance imaging (MRI) provides a powerful tool for the investigation of chemical structures in optically opaque porous media, in which chemical concentration gradients can be visualized, and diffusion and flow properties are simultaneously determined. In this paper we give an overview of the MRI technique and review theory and experiments on the formation of chemical waves in a tubular packed bed reactor upon the addition of a nonlinear chemical reaction. MR images are presented of reaction-diffusion waves propagating in the three-dimensional (3D) network of channels in the reactor, and the 3D structure of stationary concentration patterns formed via the flow-distributed oscillation mechanism is demonstrated to reflect the local hydrodynamics in the packed bed. Possible future directions regarding the influence of heterogeneities on transport and reaction are discussed.

pH oscillations and bistability in the methylene glycol–sulfite–gluconolactone reaction

The Methylene Glycol-Sulfite-Gluconolactone (MGSG) reaction is the first example of an organic-based pH oscillator. This reaction is of particular interest as it displays large amplitude oscillations in hydroxide ion accompanied by small amplitude (10(-3) V) oscillations in potential, indicating that it is not driven by redox processes. We investigate the reaction in a batch (closed) and flow (open) reactor and examine the role of the aging of the gluconolactone stock solution. The system is found to display oscillations and bistability for a wide range of flow rates and initial compositions. The experimental results are reproduced in numerical simulations in an extended model of the reaction in which the decay of the stock solution is incorporated. Finally, we analyse the features of the reaction that make it a suitable basis for the development of novel pH oscillators.

Motion and Interaction of Aspirin Crystals at Aqueous–Air Interfaces

Small-molecule amphiphiles such as aspirin have unique properties arising from a combination of an aromatic hydrophobic part and a hydrophilic part. We show that crystals of aspirin are capable of generating convective flows at the air-aqueous interface from both Marangoni effects (through weak surface activity) and capillarity (surface deformations). The flow-driven motion of millimeter-sized crystals was found to depend on the presence of other ions in solution as well as the distance and orientation of the crystals. The interactions lead to the formation of groups of two or more crystals that also underwent motion. The convective flows created by small amphiphile crystals might be exploited in the dynamic self-organization of particles at interfaces.