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Water-rock interactions in the Earth's crust often modify the pore space and permeability of rocks, soils, and sediments, changing the way fluids flow in the subsurface. Crucially, such geochemical reactions are often controlled by nano-scale processes. In this study, we use atomic force microscopy to examine dissolution rates of limestone; crucially, our measurements show that the rate of mineral dissolution within micron-size pores is much lower than the rate of dissolution on surrounding polished mineral surfaces. In addition, we use numerical simulations to show that this difference cannot be explained using a diffusion - surface reaction model. We propose that the observed variation in reaction rates could instead be due to the elevated density of reactive high curvature features on the polished surfaces. These features can strongly affect local interfacial free energy, making surfaces more prone to dissolution. As a result, polished surfaces should be more reactive than pore surfaces that have effectively been smoothed during prolonged contact with natural fluids. As standard rate experiments routinely use polished and powdered samples, our findings may help to explain the widely reported discrepancy between lab and field-based dissolution rates.

We propose an alternative method to compute the entropy production of a classical underdamped nonequilibrium system in a continuous phase space. This approach has the advantage that it is not necessary to distinguish between even and odd-parity variables. We show that the method leads to the same local entropy production as in previous studies while the differential entropy production along a stochastic trajectory turns out to be different. This demonstrates that the differential entropy production in continuous phase space systems is not uniquely defined.
See also: arxiv 1305.0655, not yet published

The tactile sensations mediated by the whisker-to-barrel cortex system allow rodents to efficiently detect and discriminate objects and surfaces. The temporal structure of whisker deflections and the temporal correlation between deflections occurring on several whiskers simultaneously vary for different tactile substrates. We hypothesize that tactile discrimination capabilities rely strongly on the ability of the system to encode different levels of inter-whisker correlations.
To test this hypothesis, we generated complex spatio-temporal patterns of whisker deflections during electrophysiological recordings in the barrel cortex, the ventro-posterior medial (VPM) nucleus of the thalamus and the trigeminal ganglion. A piezoelectric-based stimulator featuring 24 independent and fully adjustable whisker actuators was built for this purpose (Jacob et al., 2010).
Using this stimulator in anesthetized rats, we have previously shown that cortical neurons exhibit direction selectivity to the apparent motion of a multivibrissal stimulus (i.e. an emerging property of the global stimulus), uncorrelated to the local direction of individual whiskers (Jacob et al. 2008). Since a certain level of multiwhisker integration has been reported in the VPM, the nucleus relaying tactile information to the barrel cortex, we showed that emergent properties of multiwhisker stimulations are already coded by VPM neurons although to a lesser degree than in cortex (Ego-Stengel et al., 2012).
Finally, we applied a reverse correlation approach to this problem by using Gaussian white noise stimulation on 24 whiskers and progressively varying the level of temporal correlation among them. Based on spike-triggered analysis for various levels of inter-whisker correlation, our recent findings (Estebanez et al., 2012) show that neuronal cortical networks implement coexisting coding schemes to cope with the varying statistics of the tactile sensory world. We propose a simple and comprehensive framework that not only accounts for most of the previous reported phenomenology of multiwhisker interactions but also provides a physiological role for this functional selectivity in terms of local contrast and global motion detection.