The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application Surprises from single-particle imaging of passive and active diffusion

The transport of matter and energy is fundamental in nature and biology. It can occur by passive diffusion and can also be active. Predicated on fluorescence imaging at the single-particle level, this talk describes quantitative studies of how this can happen, and presents examples of how nontrivial findings emerge from observing the rare events that underly the overall ensemble-averaged distribution. For example, in living cells, we find that transportation efficiency problems bear a provocative parallel with polymer chain trajectories with their spatial extent, and with jammed matter in their time evolution. A picture emerges in which simple experiments, performed at single-particle an

How to better focus waves by considering symmetry and information loss

We amend the general belief that waves with extended spherical wavefront focus at their center of curvature. Instead, when the spherical symmetry of waves is broken by propagating them through a finite aperture along an average direction, the forward/backward symmetry is broken and the focal volume shifts its center backward along that direction. The extent of this focal shift increases as smaller apertures are used, up to the point that the nominal focal plane is out of focus. Furthermore, the loss of axial symmetry with noncircular apertures causes distinct focal shifts in distinct axial planes, and the resulting astigmatism possibly degrades the axial focusing resolution. Using experiments and simulations, focal shift with noncircular apertures is described for classical and temporal focusing. The usefulness of these conclusions to improve imaging resolution is demonstrated in a highr-esolution optical microscopy application, namely line-temporal focusing microscopy. These conclusions follow from fundamental symmetries of the wave geometry and matter for an increasing number of emerging optical techniques. This work offers a general framework and strategy to understand and improve virtually any wave-based application whose efficacy depends on optimal focusing and may be helpful when information is transmitted by waves in applications from electromagnetic communications, to biological and astronomical imaging, to lithography and even warfare

Management strategies for saving water and increasing its productivity in lowland rice-based ecosystems

A new method of noncontact temperature measurement in microliter-sized volumes is demonstrated, based on the temperature sensitivity of the fluorescence lifetime of rhodamine-G when it is attached to a DNA oligomer. As temperature changes, the spacing between the fluorescent dye and a designed sequence of DNA bases is modulated by conformation changes of the DNA chain, and as a result the ability of dye molecules to fluoresce is also modulated according to differential quenching by bases on the DNA. In the system that we studied, the temperature sensitivity of the fluorescence lifetime was 36???42 ps/??C depending on specific solution conditions. Although this strategy of temperature measurement is demonstrated using a specific sequence of DNA, it can also be generalized to a dye attached to any other intrinsic quencher of fluorescence whose conformation changes with temperature

A surface forces platform for dielectric measurements

Methods are described to implement dielectric spectroscopy (frequency range 10(-1)-10(6) Hz) within a surface forces apparatus by using as electrodes silver sheets on the backside of mica. These methods are applied to study the competitive effects of surface alignment, confinement, and shear field on 5CB (5-cyanobiphenyl), a nematic liquid crystal at the experimental temperature of 25degreesC. In the planar orientation, films could be squeezed to a minimum thickness of approximate to5 Angstrom, the molecule's thickness. In the perpendicular (homeotropic) orientation, films could be squeezed to approximate to25 Angstrom, the expected thickness of the head-to-tail 5CB dimer. It was difficult to discuss responses at f >10(5) Hz quantitatively because the peak was not visible in the experimental frequency window. Nonetheless, the onset of the relaxation mode for the planar oriented molecules appears at higher frequency than for the homeotropic orientation. A slower relaxation mode, peaked at f approximate to 10 Hz, was assigned to electrode polarization due to the mobility of trace ions within the 5CB samples although these samples were >99.7% pure. The peak frequency was a factor of 3 slower with homeotropic than planar alignment and, in both cases, independent of film thickness except when the film thickness exceeded 10 mum. This was explained using a simple model based on the assumption that trace ions move to oppositely charged electrodes to form electric double layers. A small influence of shear on the dielectric response was observed but only when the dielectric response was measured at the same frequency as the large-amplitude shear deformation. Also described is the use of capacitance to measure force-distance profiles.open8

Dielectric response of polymer films confined between mica surfaces

The thin-film dielectric response of organic films confined within a surface forces apparatus (SFA) and also between parallel sheets of atomically smooth mica is reported for the first time. Analysis is presented to infer dielectric properties of the organic film from the measured capacitance of the total system: sample, and mica sheets intervening between sample and electrodes. Measurements concerned the frequency dependence of normal-mode dielectric relaxation of cis-polyisoprene having dipoles aligned in the same direction along the chain backbone. We find that in thin-film geometries the peak frequency, f(peak), of normal mode dielectric loss (epsilon") is moderately lower than for bulk samples and that, more important, the expected terminal tail, observed in the bulk sample (epsilon"proportional to f for f < f(peak)), is not observed even at the lowest frequency examined. Thus the slow normal mode distribution is much broader and the terminal relaxation time is much longer for chains in the thin layers. These dielectric features are attributed to spatial constraints on global chain motion in the thin layers and also to adsorption of chains on mica surfaces when the layer thickness is comparable to the unperturbed chain dimension. Independent measurements of shear relaxation, performed using a SFA modified for measurement of dynamical mechanical shear rheology, found a tremendously retarded viscoelastic response relative to bulk samples. There is the possibility that the broad distribution of the dielectric response of individual polymer chains may correspond to the observed retarded viscoelastic relaxation. However, we cannot rule out the other possibility that the dielectrically detected relaxation of individual chains is still faster than the terminal viscoelastic relaxation and that the latter thus corresponds to the collective motion of many confined chains.open333

Machine learning assembly landscapes from particle tracking data

Bottom-up self-assembly offers a powerful route for the fabrication of novel structural and functional materials. Rational engineering of self-assembling systems requires understanding of the accessible aggregation states and the structural assembly pathways. In this work, we apply nonlinear machine learning to experimental particle tracking data to infer low-dimensional assembly landscapes mapping the morphology, stability, and assembly pathways of accessible aggregates as a function of experimental conditions. To the best of our knowledge, this represents the first time that collective order parameters and assembly landscapes have been inferred directly from experimental data. We apply this technique to the nonequilibrium self-assembly of metallodielectric Janus colloids in an oscillating electric field, and quantify the impact of field strength, oscillation frequency, and salt concentration on the dominant assembly pathways and terminal aggregates. This combined computational and experimental framework furnishes new understanding of self-assembling systems, and quantitatively informs rational engineering of experimental conditions to drive assembly along desired aggregation pathways. © 2015 The Royal Society of Chemistryope

Reincarnations of the phase separation problem

Phase separation is familiar and useful, yet opportunities to manipulate it are surprisingly subtle and complex.11Nsciescopu