The mean acoustic intensity, m, in the quiet Sun seems to increase with depth, as shown in Fig. 3. The depth dependence of m may be related to the variation of acoustic absorption with depth and the fraction of the acoustic spatiotemporal spectrum that is detected in viewing different depths. (5) The spatial resolution of constructed images decreases with depth. The time-distance relations flatten with increasing depth, so the phase gradient over the observing annulus decreases. The difference in phases used to image adjacent spatial points becomes small, and a point in space is imaged with a broad point-spread function. Another cause is that shorter-wavelength modes cannot be detected in viewing a deeper region.Finally, we mention some important questions that call for further study. What is the degree of cancellation of signals from points other than the target point in this phased detection? What are the horizontal and vertical spatial resolutions corresponding to this 'computational acoustic lens'? What is the correct interpretation of the mean intensity, m, and its apparent variation with depth? Can this method reconstruct the complex index of refraction of acoustic waves at each target point, using the phase information in addition to the intensity? Finally, could one use this method to detect active regions before they emerge, or active regions at the other side of the Sun?Ⅺ

We report gas and water flow measurements through microfabricated membranes in which aligned carbon nanotubes with diameters of less than 2 nanometers serve as pores. The measured gas flow exceeds predictions of the Knudsen diffusion model by more than an order of magnitude. The measured water flow exceeds values calculated from continuum hydrodynamics models by more than three orders of magnitude and is comparable to flow rates extrapolated from molecular dynamics simulations. The gas and water permeabilities of these nanotube-based membranes are several orders of magnitude higher than those of commercial polycarbonate membranes, despite having pore sizes an order of magnitude smaller. These membranes enable fundamental studies of mass transport in confined environments, as well as more energy-efficient nanoscale filtration.

Solids dispersed in a drying drop will migrate to the edge of the drop and form a solid ring. This phenomenon produces ringlike stains and occurs for a wide range of surfaces, solvents, and solutes. Here we show that the migration is caused by an outward flow within the drop that is driven by the loss of solvent by evaporation and geometrical constraint that the drop maintain an equilibrium droplet shape with a fixed boundary. We describe a theory that predicts the flow velocity, the rate of growth of the ring, and the distribution of solute within the drop. These predictions are compared with our experimental results.

Biological pores regulate the cellular traffic of a large variety of solutes, often with high selectivity and fast flow rates. These pores share several common structural features: the inner surface of the pore is frequently lined with hydrophobic residues, and the selectivity filter regions often contain charged functional groups. Hydrophobic, narrow-diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform. Previous studies demonstrated that carbon nanotube pores can support a water flux comparable to natural aquaporin channels. Here, we investigate ion transport through these pores using a sub-2-nm, aligned carbon nanotube membrane nanofluidic platform. To mimic the charged groups at the selectivity region, we introduce negatively charged groups at the opening of the carbon nanotubes by plasma treatment. Pressure-driven filtration experiments, coupled with capillary electrophoresis analysis of the permeate and feed, are used to quantify ion exclusion in these membranes as a function of solution ionic strength, pH, and ion valence. We show that carbon nanotube membranes exhibit significant ion exclusion that can be as high as 98% under certain conditions. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, whereas steric and hydrodynamic effects appear to be less important.biomimetic platform ͉ ion channel ͉ ion transport ͉ nanofiltration I on transport across cellular membranes is essential to many of life's processes, such as electrical signaling in nerves, muscles, and synapses or cell's maintenance of homeostatic balance. Biological systems achieve rapid, selective, and ultraefficient transmembrane mass transport by employing a large variety of specialized protein channels of nanometer or subnanometer size (1). High-resolution x-ray structures, protein sequencing, targeted mutations, and biophysical characterizations have provided new insight on the link between nanochannel protein architecture, transport rates, selectivity, and gating properties. Interestingly, these studies have shown that membrane nanochannels share several common features. For example, aquaporins (2, 3), proton channels (4, 5), and ion channels (6-11) all have relatively narrow and hydrophobic pore regions. By contrast, the selectivity filter regions of membrane ion channels are enriched with charged residues.Despite the enormous progress made in recent decades, the complex macromolecular nature of these biological machines still complicates our understanding of the underlying mechanisms responsible for fast mass transport, selectivity, gating, and the functional role of hydrophobic pore lining and charged functionalities. Thus, it is important to create simplified, biomimetic nanochannels that could help to clarify the physics of ion permeation at the nanoscale, as well as create the next generation of membranes that employ efficient molecul...

Dielectrophoretic trapping of molecules is typically carried out using metal electrodes to provide high field gradients. In this paper we demonstrate dielectrophoretic trapping using insulating constrictions at far lower frequencies than are feasible with metallic trapping structures because of water electrolysis. We demonstrate that electrodeless dielectrophoresis (EDEP) can be used for concentration and patterning of both single-strand and double-strand DNA. A possible mechanism for DNA polarization in ionic solution is discussed based on the frequency, viscosity, and field dependence of the observed trapping force.

We have used the combination of single-molecule Fö rster resonance energy transfer and kinetic synchrotron radiation circular dichroism experiments to probe the conformational ensemble of the collapsed unfolded state of the small cold shock protein CspTm under near-native conditions. This regime is physiologically most relevant but difficult to access experimentally, because the equilibrium signal in ensemble experiments is dominated by folded molecules. Here, we avoid this problem in two ways. One is the use of single-molecule Fö rster resonance energy transfer, which allows the separation of folded and unfolded subpopulations at equilibrium and provides information on long-range intramolecular distance distributions. From experiments with donor and acceptor chromophores placed at different positions within the chain, we find that the distance distributions in unfolded CspTm agree surprisingly well with a Gaussian chain not only at high concentrations of denaturant, where the polypeptide chain is expanded, but also at low denaturant concentrations, where the chain is collapsed. The second, complementary approach is synchrotron radiation circular dichroism spectroscopy of collapsed unfolded molecules transiently populated with a microfluidic device that enables rapid mixing. The results indicate a ␤-structure content of the collapsed unfolded state of Ϸ20% compared with the folded protein. This suggests that collapse can induce secondary structure in an unfolded state without interfering with long-range distance distributions characteristic of a random coil, which were previously found only for highly expanded unfolded proteins.Gaussian chain ͉ microfluidic mixing ͉ protein folding ͉ random coil ͉ secondary structure W ith the discovery of small proteins that fold in the absence of populated intermediates (1), our quantitative understanding of the elementary properties of protein folding reactions has made significant advances, including the structural characterization of transition states for folding (2) and the prediction of folding rates from native structure (3-5). One of the most severe limitations for the further development of these approaches is our ignorance about the energetic or structural properties of unfolded † † states of proteins. Because of the structural heterogeneity and complexity of the ensembles of conformations populated by unfolded proteins, their experimental characterization has proven extremely difficult. Traditional methods, such as small-angle scattering techniques (6), provide only global physical properties, e.g., the radius of gyration. In some cases, more detailed structural information can be obtained from NMR (7-10), but these studies usually provide information about the denatured state only under nonnative conditions, typically in the presence of large concentrations of denaturant, or through severe destabilization of the native state induced by covalent modification or mutations. The most interesting and physiologically relevant situation, however, is that of an unfolded sta...

In order to investigate the behavior of single molecules under conditions far from equilibrium, we have coupled a microfabricated laminar-flow mixer to a confocal optical system. This combination enables time-resolved measurement of Förster resonance energy transfer after an abrupt change in solution conditions. Observations of a small protein show the evolution of the intramolecular distance distribution as folding progresses. This technique can expose subpopulations, such as unfolded protein under conditions favoring the native structure, that would be obscured in equilibrium experiments.

The effect of confinement on the dynamics of polymers was studied by observing the transient extension and relaxation of single DNA molecules as they interacted with obstacles in a specially designed thin slit. Viscous drag was found to increase with the degree of confinement, which we interpret in terms of hydrodynamic screening by the planar surfaces of the slit. Since the DNA was driven by an electrophoretic force, the experimental data support the notion that an electric field acts on a tethered polyelectrolyte equivalently to a hydrodynamic flow. [S0031-9007(98)