Completely renewable energy can be produced by using water solutions of different salinity, like river water and sea water. Many different methods are already known, but development is still at prototype stage. Here I report a novel method, based on electric double-layer capacitor technology. Two porous electrodes, immersed in the salt solution, constitute a capacitor. It is first charged, then the salt solution is brought into contact with fresh water. The electrostatic energy increases as the salt concentration of the solution is reduced due to diffusion. This device can be used to turn sources of salinity difference into completely renewable sources of energy. An experimental demonstration is given, and performances and possible improvements are discussed.

We describe the use of a bright-field microscope for dynamic light scattering experiments on weakly scattering samples. The method is based on collecting a time sequence of microscope images and analyzing them in the Fourier space to extract the characteristic time constants as a function of the scattering wave vector. We derive a theoretical model for microscope imaging that accounts for ͑a͒ the three-dimensional nature of the sample, ͑b͒ the arbitrary coherence properties of the light source, and ͑c͒ the effect of the finite numerical aperture of the microscope objective. The model is tested successfully against experiments performed on a colloidal dispersion of small spheres in water, by means of the recently introduced differential dynamic microscopy technique ͓R. Cerbino and V. Trappe, Phys. Rev. Lett. 100, 188102 ͑2008͔͒. Finally, we extend our model to the class of microscopy techniques that can be described by a linear space-invariant imaging of the density of the scattering centers, which includes, for example, dynamic fluorescence microscopy.

The tethered particle motion (TPM) technique involves an analysis of the Brownian motion of a bead tethered to a slide by a single DNA molecule. We describe an improved experimental protocol with which to form the tethers, an algorithm for analyzing bead motion visualized using differential interference contrast microscopy, and a physical model with which we have successfully simulated such DNA tethers. Both experiment and theory show that the statistics of the bead motion are quite different from those of a free semiflexible polymer. Our experimental data for chain extension versus tether length fit our model over a range of tether lengths from 109 to 3477 base pairs, using a value for the DNA persistence length that is consistent with those obtained under similar solution conditions by other methods. Moreover, we present the first experimental determination of the full probability distribution function of bead displacements and find excellent agreement with our theoretical prediction. Our results show that TPM is a useful tool for monitoring large conformational changes such as DNA looping.

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Electrical energy can be obtained from the controlled mixing of fresh (river) and saline (sea) water. Existing technologies such as pressure retarded osmosis and reverse electrodialysis make use of ionexchange membranes which must be crossed by either the water or the ions. Recently a new physical principle has been experimentally demonstrated, which allows extraction of electrical energy without making use of membranes, based on the temporary storage of ions inside two porous electrodes kept at different electrical potentials, and the repeatable expansion/contraction of the electrostatic double layers formed inside the electrodes upon changing the salt concentration [D. Brogioli, Phys. Rev. Lett., 2009, 103, 058501]. To make further investigations and to improve the energy recovery, we developed a simple prototype cell of much larger dimensions. Because of the larger dimensions (thus higher currents), testing is more facile, while this design can be the basis for further scaling-up of this technology. In order to reduce the internal resistance of the cell, the electrodes are no longer placed side-by-side, but parallel to one another, separated only by a 250 mm-thick open spacer channel to form a ''sandwich''-like flow cell. In a lab-scale experimental stack consisting of 8 such cells (with outer dimensions 6 Â 6 Â 1 cm 3) we extract about 2 J per charging/discharging cycle in 500 mM/1 mM NaCl salt solution, an amount which is 20 times higher per cycle per unit electrode mass than previously obtained. The extracted energy increases with the operating voltage, in line with predictions of the Gouy-Chapman-Stern model for double layer formation.

Combining two solutions of different composition releases the Gibbs free energy of mixing. By using engineered processes to control the mixing, chemical energy stored in salinity gradients can be harnessed for useful work. In this critical review, we present an overview of the current progress in salinity gradient power generation, discuss the prospects and challenges of the foremost technologies - pressure retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CapMix) and provide perspectives on the outlook of salinity gradient power generation. Momentous strides have been made in technical development of salinity gradient technologies and field demonstrations with natural and anthropogenic salinity gradients (for example, seawater-river water and desalination brine-wastewater, respectively), but fouling persists to be a pivotal operational challenge that can significantly ebb away cost-competitiveness. Natural hypersaline sources (e.g., hypersaline lakes and salt domes) can achieve greater concentration difference and, thus, offer opportunities to overcome some of the limitations inherent to seawater-river water. Technological advances needed to fully exploit the larger salinity gradients are identified. While seawater desalination brine is a seemingly attractive high salinity anthropogenic stream that is otherwise wasted, actual feasibility hinges on the appropriate pairing with a suitable low salinity stream. Engineered solutions are foulant-free and can be thermally regenerative for application in low-temperature heat utilization. Alternatively, PRO, RED, and CapMix can be coupled with their analog separation process (reverse osmosis, electrodialysis, and capacitive deionization, respectively) in salinity gradient flow batteries for energy storage in chemical potential of the engineered solutions. Rigorous techno-economic assessments can more clearly identify the prospects of low-grade heat conversion and large-scale energy storage. While research attention is squarely focused on efficiency and power improvements, efforts to mitigate fouling and lower membrane and electrode cost will be equally important to reduce levelized cost of salinity gradient energy production and, thus, boost PRO, RED, and CapMix power generation to be competitive with other renewable technologies. Cognizance of the recent key developments and technical progress on the different technological fronts can help steer the strategic advancement of salinity gradient as a sustainable energy source.

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The ''capacitive mixing'' (CAPMIX) technique is aimed at the extraction of energy from the salinity difference between the sea and rivers. It is based on the voltage rise that takes place at the electrodes when changing the salt concentration of the solution in which the two electrodes are dipped. In this paper, we focus on activated carbon electrodes, produced with various methods and treatments, and we measure their behaviour in CAPMIX experiments. We find that they behave as polarizable electrodes only on time scales of the order of minutes, while on longer time scales they tend to move to a specific ''spontaneous'' potential, likely due to chemical redox reactions. This analysis sheds light on the charge leakage, i.e. the loss of the stored charge due to undesired chemical reactions, which is one of the main hurdles of the CAPMIX technique when performed with activated carbon electrodes. We show that the leakage finds its origin in the tendency of the electrode to move to its spontaneous potential. Our investigation allows us to completely get rid of the leakage and demonstrates the striking result that CAPMIX cycles can be performed without an external power supply.