Mobility of reactants and nearby solvent is more rapid than Brownian diffusion during several common chemical reactions when the energy release rate exceeds a threshold. Screening a family of 15 organic chemical reactions, we demonstrate the largest boost for catalyzed bimolecular reactions, click chemistry, ring-opening metathesis polymerization, and Sonogashira coupling. Boosted diffusion is also observed but to lesser extent for the uncatalyzed Diels-Alder reaction, but not for substitution reactions SN1 and SN2 within instrumental resolution. Diffusion coefficient increases as measured by pulsed-field gradient nuclear magnetic resonance, whereas in microfluidics experiments, molecules in reaction gradients migrate “uphill” in the direction of lesser diffusivity. This microscopic consumption of energy by chemical reactions transduced into mechanical motion presents a form of active matter.

Günther et al. report that their control experiment using randomized magnetic field gradient sequences disagreed with findings we had reported using linear gradients. However, we show that measurements in our laboratory are consistent using both methods.

Thermophoresis refers to the motion of particles under a temperature gradient and it is one of the particle manipulation techniques. Regarding the thermophoresis of particles in liquid media, however, many open questions still remain, especially the role of the interfacial effect. This work reports on a systematic experimental investigation of surfactant effects, especially the induced interfacial effect, on the thermophoresis of colloids in aqueous solutions via a microfluidic approach. Two kinds of commonly used surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), are selected and the results show that from relatively large concentrations, the two surfactants can greatly enhance the thermophilic mobilities. Specifically, it is found that the colloid-water interfaces modified with more polar end groups can potentially lead to a stronger thermophilic tendency. Due to the complex effects of surfactants, further theoretical model development is needed to quantitatively describe the dependence of thermophoresis on the interface characteristics.

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Rotational diffusion processes are correlated with nanoparticle visualization and manipulation techniques, widely used in nanocomposites, nanofluids, bioscience, and so on. However, a systematical methodology of deriving this diffusivity is still lacking. In the current work, three molecular dynamics (MD) schemes, including equilibrium (Green-Kubo formula and Einstein relation) and nonequilibrium (Einstein-Smoluchowski relation) methods, are developed to calculate the rotational diffusion coefficient, taking a single rigid carbon nanotube in fluid argon as a case. We can conclude that the three methods produce same results on the basis of plenty of data with variation of the calculation parameters (tube length, diameter, fluid temperature, density, and viscosity), indicative of the validity and accuracy of the MD simulations. However, these results have a non-negligible deviation from the theoretical predictions of Tirado et al. [J. Chem. Phys. 81, 2047 (1984)], which may come from several unrevealed factors of the theory. The three MD methods proposed in this paper can also be applied to other situations of calculating rotational diffusion coefficient.

The nanoparticle orientation in fluid systems can be correlated with the rotational diffusion and is widely used to tune the physical properties of functional materials. In the current work, the controllability of the orientation of a single rigid carbon nanotube in a fluid is investigated by imposing a linear shear flow. Molecular dynamics simulations reveal three forms of anomalous behavior: (i) “Aligned orientation” when the nanotube oscillates around a particular direction which is close to the flow direction at a small angle of about 10° in the velocity-gradient plane; (ii) “Interrupted orientation” when the oscillation is interrupted by a 360° rotation now and then; (iii) “Random orientation” when 360° rotations dominate with the rotational direction coinciding with the local fluid flow direction. The orientation order is a function of the Peclet number (Pe). The results show that the correlation between Pe and the orientation order from the two-dimensional model does not apply to the three-dimensional cases, perhaps due to some anomalous behavior and cross-section effects. This work provides clear pictures of the nanoparticle movement that can be used to guide particle manipulation techniques.

Phase separation is familiar and useful, yet opportunities to manipulate it are surprisingly subtle and complex. Two just-published papers 1,2 remind us that in approaching any new scientific problem, there are generally three stages of reaction. Our first impression is of difference and strangeness. But once superficial dissimilarities are pierced, our feeling becomes the opposite extreme. Everything now appears fundamentally the same. All that we can now see are the underlying commonalities, with different forms according to their settings. Only in the third stage does real knowledge begin. We look anew for differences, not this time for those obvious elements that hit the eyes of newcomers, but rather for subtleties whose importance structures the problem. Phase separation is an evergreen subject that reinvents itself continually. Starting early on, scientists and engineers sought to control items such as steel microstructure and to understand workaday items such as why oil floats on soup, leading in the 19th and early 20th century to focus on nucleation and growth 3. Then came a surprise-the identification of spinodal decomposition 4 ; it was followed by an era of exploring the spinodal decomposition formalism in many manifestations, a pattern that to some extent continues today. Independently, explosive advances follow from progress in understanding critical points, an accomplishment recognized by a Nobel Prize in the latter part of the 20th century 5. Surveying the ensuing scientific literature one finds that the phrase "phase separation" now is used progressively less often. During the 21st century, this loss is balanced by increasing the use of the phrase "self-assembly". For experimentalists, much of the distinction between these concepts can be semantic. Both concepts, phase separation and self-assembly, share the concern with understanding how order appears from disorder. A new coarsening mechanism in phase separation 1 and bicontinuous nanoparticle gels 2 are reported from the groups of Hajime Tanaka at the University of Tokyo and Yun Liu at the University of Delaware/National Institute of Standards and Technology, respectively. On first reading, the subject seems to differ from what professors teach students about phase separation-where are the phase diagrams, tie lines, and other standard thermodynamic quantities? These studies, dwelling instead on how processes evolve in time, seem alien to the textbook 3 view. On the second reading, the phase-separated structures are familiar-coarsening 1 , a standard feature of nucleation and growth, and bicontinuous structures 2 , standard for spinodal decomposition, so from this perspective, readers can feel comfortable. It requires third reading to see where these interesting studies go beyond the standard view. Both studies exemplify how a mere difference in mobility can influence even the long-time structural appearance of two separating components-one of them relatively fast, the other relatively slow. This problem doesn't present itself in the usual textbo...

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheological behaviors of a monoatomic fluid governed by the Lennard-Jones potential. Both steady Couette and oscillatory shear flows are investigated. Shear thinning and normal stress effects are observed in the steady Couette flow simulations. The radial distribution function is calculated at different shear rates to exhibit the change of the microscopic structure of molecules due to shear. We observe that for a larger shear rate the repulsion between molecules is more powerful while the attraction is weaker, and the above phenomena can also be confirmed by the analyses of the potential energy. By applying an oscillatory shear to the system, several findings are worth mentioning here: First, the phase difference between the shear stress and shear rate increases with the frequency. Second, the real part of complex viscosity first increases and then decreases while the imaginary part tends to increase monotonically, which results in the increase of the proportion of the imaginary part to the real part with the increasing frequency. Third, the ratio of the elastic modulus to the viscous modulus also increases with the frequency. These phenomena all indicate the appearance of viscoelasticity and the domination of elasticity over viscosity at high oscillation frequency for Lennard-Jones fluids.