Quantifying the non-equilibrium activity of an active colloid

Active matter systems exhibit rich emergent behavior due to constant injection and dissipation of energy at the level of individual agents. We characterize the dissipation of single active colloids.

Tunable dynamics of microtubule-based active isotropic gels

We investigate the dynamics of an active gel of bundled microtubules (MTs) that is driven by clusters of kinesin molecular motors. Upon the addition of ATP, the coordinated action of thousands of molecular motors drives the gel to a highly dynamical turbulent-like state that persists for hours and is only limited by the stability of constituent proteins and the availability of the chemical fuel. We characterize how enhanced transport and emergent macroscopic flows of active gels depend on relevant molecular parameters, including ATP, kinesin motor and depletant concentrations, MT volume fraction, as well as the stoichiometry of the constituent motor clusters. Our results show that the dynamical and structural properties of MT-based active gels are highly tunable. They also indicate existence of an optimal concentration of molecular motors that maximize far-from-equilibrium activity of active isotropic MT gels.

Corrosion Behavior of Spent MTR Fuel Elements in a Drowned Salt Mine Repository

Spent MTR fuel from German Material Test Reactors will not be reprocessed, but stored in a final salt repository in the deep geologic underground. Fuel elements will be placed in POLLUX containers, which are assumed to resist the corrosive attack of an accidentally formed concentrated salt brine for about 500 years. After a container failure the brine would contact the fuel element, corrode the aluminum plating and possibly leach radionuclides from the fuel.A source term for the calculation of radionuclide mobilization results from the investigation of the behavior of MTR fuel in this scenario, which has to be considered for the long- term safety analysis of a deep mined rock salt repository.Experiments with the different plating materials show that the considered aluminum alloys will not resist the corrosive attack of a brine solution, especially in the presence of iron, under the conditions in a drowned salt mine repository. Although differences in the corrosion rates of about two orders of magnitude were observed when applying different parameter sets, the deterioration must be considered to be almost instantaneous in geological terms.Radionuclides are mobilized from irradiated MTR fuel, when the “meat” of the fuel element becomes accessible to the brine solution. It seems, however, that the radionuclides are effectively trapped by the aluminum hydroxide formed, as the activity concentrations in the brine solution soon reach a constant level with the progressing corrosion of the cladding aluminum. In the presence of iron a more significant initial release was observed, but also in this case an equilibrium activity seems to be reached as a consequence of radionuclide trapping.

Ion Activities and Electrochemical Gradients in the Mealworm Rectal Complex

Ion activities and potential differences in cellular and extracellular compartments of the rectal complex of Tenebrio molitor L. larvae have been recorded simultaneously using double-barrelled ion-sensitive microelectrodes. On average, the tubule lumen (TL) was 44mV positive to the haemolymph. Values of aK in the posterior rectal complex exceeding 2700 mmol l−1 were measured, sufficient to account for much of the osmolality of 6.8 osmol kg−1 driving uptake of water from the rectal lumen. The mean value of 797 mmol l−1 exceeded the Nernst equilibrium activity more than 75-fold, indicating active transport of K+. Intracellular potassium activities in the tubules (153 mmol l−1) were high relative to the values in other insect cells, but moderate relative to TL values. Tubule lumen Na+ activities as high as 400 mmol l−1 and pH values of 6.8 were well above the equilibrium values of 11 mmol l−1 and 7.9, respectively, indicating active transport of these cations as well. The ease and frequency of impaling a perinephric space (PNS) surrounding the tubules established it as a functional compartment. On average, the PNS was 22 mV negative to the haemolymph. Potassium activities in the PNS were close to electrochemical equilibrium with the haemolymph, whereas mean aNa and pH were reduced fivefold and 0.5 units, respectively, below the corresponding Nernst equilibrium values. The results suggest that cations move from haemolymph to PNS, and that the PNS is the immediate source for cation transport into the tubule lumen. Cl− was close to electrochemical equilibrium with the haemolymph in both compartments, and presumably enters the tubule lumen as a passive consequence of positive potential differences (PDs) in the tubule lumen.

Predicting exchangeable cation distributions in soil by using exchange coefficients and solution activity ratios

A simple model which predicts the distribution of surface-applied chemicals in soil following leaching is described. The applied cation is considered to reach equilibrium between the solution and exchange phases prior to moving into the underlying soil layer. The distribution between the solution and exchange phases is defined by the Gapon relationship. The Gapon exchange coefficient is derived from exchange isotherms determined using a batch technique. Movement of the applied cation through successive layers and cation exchange with the soil continues until the activity ratio in solution equals the equilibrium activity ratio determined from the exchange isotherm. The applied cation remaining in solution is then considered to move 'freely'. A number of physical and chemical processes have been omitted in developing the model to retain simplicity and to ensure that the required parameters can readily be obtained using simple and well established laboratory procedures. In particular, only a binary cation exchange system is considered, and it is assumed that the rate of exchange is instantaneous. The inputs to the model are the amount of cation added, the Gapon exchange coefficient and corresponding solution activity ratio, the equilibrium activity ratio, the initial exchangeable and solution cation concentrations, and the average volumetric moisture content of soil during leaching. The simple model satisfactorily predicted the distribution of exchangeable K+ and Mg2+, and percentage of applied K+ or Mg2+ leaching 'freely' in the soil solution in homogeneous columns of soil having varying texture, clay mineralogy, and initial cation composition.