The applicability of the Hertz–Knudsen equation to the evolution of droplets at the nanoscale was investigated upon analysis of existing molecular dynamics (MD) simulations ( Holyst; Phys. Rev. Lett. 2008, 100, 055701; Yaguchi; J. Fluid Sci. Technol. 2010, 5, 180–191; Ishiyama; Phys. Fluids 2004, 16, 2899–2906). The equation was found satisfactory for radii larger than ∼4 nm. Concepts of the Gibbs equimolecular dividing surface and the surface of tension were utilized in order to accommodate the surface phase density and temperature profiles, clearly manifesting at the nanoscale. The equimolecular dividing surface was identified as the surface of the droplet. A modification to the Tolman formula was proposed in order to describe surface tension for droplet radii smaller than ∼50 nm. We assumed that the evaporation coefficient for a system in and out of equilibrium may differ. We verified that this difference might be attributed to surface temperature change only. The empirical dependencies of the evaporation coefficient and the surface tension for a flat interface, of liquid Ar in Ar gas at equilibrium, at the nanoscale, upon temperature was taken from existing MD data. Two parametrizations of the Hertz–Knudsen equation were proposed: (i) one using the off-equilibrium condensation coefficient and the effective density and (ii) another one using the effective density and the temperature at the interface. The second parametrization leads to an approximate solution of the Hertz–Knudsen equation requiring no free parameters. Such a solution is suitable for experimental use at the nanoscale if only the temperature of the droplet (core) can be measured.

A stable and efficient surface-enhanced Raman scattering (SERS) substrate for neurotransmitter and cholinergic neurotransmission precursor detection was obtained by silver nanoparticle (AgNP) electrodeposition onto tin-doped indium oxide (ITO) using cyclic voltammetry. The size and surface coverage of the deposited AgNPs were controlled by changing the scan rate and the number of scans. The SERS performance of these substrates was analyzed by studying its reproducibility, repeatability and signal enhancement measured from p-aminothiophenol (p-ATP) covalently bonded to the substrate. We compared the SERS performance for samples with different Ag particle coverage and particle sizes. The performance was also compared with a commercial substrate. Our substrates exhibited a SERS enhancement factor of around 10⁷ for p-ATP which is three orders of magnitude larger than for the commercial substrate. Apart from this high enhancement effect the substrate also shows extremely good reproducibility. The average spectral correlation coefficient (Γ) is 0.96. This is larger than for the commercial substrate (0.85) exhibiting a much lower SERS signal intensity. Finally, the application of our substrates as SERS bio-sensors was demonstrated with the detection of the neurotransmitters acetylcholine, dopamine, epinephrine and choline, the precursor for acetylcholine. The intensive SERS spectra observed for low concentrations of choline (2 × 10⁻⁶ M), acetylcholine (4 × 10⁻⁶ M), dopamine (1 × 10⁻⁷ M) and epinephrine (7 × 10⁻⁴ M) demonstrated the high sensitivity of our substrate. The high sensitivity and fast data acquisition make our substrates suitable for testing physiological samples.

Evaporation is ubiquitous in nature. This process influences the climate, the formation of clouds, transpiration in plants, the survival of arctic organisms, the efficiency of car engines, the structure of dried materials and many other phenomena. Recent experiments discovered two novel mechanisms accompanying evaporation: temperature discontinuity at the liquid–vapour interface during evaporation and equilibration of pressures in the whole system during evaporation. None of these effects has been predicted previously by existing theories despite the fact that after 130 years of investigation the theory of evaporation was believed to be mature. These two effects call for reanalysis of existing experimental data and such is the goal of this review. In this article we analyse the experimental and the computational simulation data on the droplet evaporation of several different systems: water into its own vapour, water into the air, diethylene glycol into nitrogen and argon into its own vapour. We show that the temperature discontinuity at the liquid–vapour interface discovered by Fang and Ward (1999 Phys. Rev. E 59 417–28) is a rule rather than an exception. We show in computer simulations for a single-component system (argon) that this discontinuity is due to the constraint of momentum/pressure equilibrium during evaporation. For high vapour pressure the temperature is continuous across the liquid–vapour interface, while for small vapour pressures the temperature is discontinuous. The temperature jump at the interface is inversely proportional to the vapour density close to the interface. We have also found that all analysed data are described by the following equation: da/dt = P₁/(a + P₂), where a is the radius of the evaporating droplet, t is time and P₁ and P₂ are two parameters. P₁ = −λΔT/(q_effρ_L), where λ is the thermal conductivity coefficient in the vapour at the interface, ΔT is the temperature difference between the liquid droplet and the vapour far from the interface, q_eff is the enthalpy of evaporation per unit mass and ρ_L is the liquid density. The P₂ parameter is the kinetic correction proportional to the evaporation coefficient. P₂ = 0 only in the absence of temperature discontinuity at the interface. We discuss various models and problems in the determination of the evaporation coefficient and discuss evaporation scenarios in the case of single- and multi-component systems.

Taylor Dispersion Analysis (TDA) has been performed for analytes moving at high flow rates in long, coiled capillaries. A thin injection zone of the analyte is stretched by the flow and final distribution of concentration of the analyte at the end of the capillary has the Gaussian shape. The high flow rates in coiled capillary generate vortices. They convectively mix the analyte across the capillary. This mixing reduces the width of the Gaussian distribution several times in comparison to the width obtained in a straight capillary in standard TDA. We have determined an empirical, scaling equation for the width as a function of the flow rate, molecular diffusion coefficient of the analyte, viscosity of the carrier phase, internal radius of the cylindrical capillary, and external radius of the coiled capillary. This equation can be used for different sizes of capillaries in a wide range of parameters without an additional calibration procedure. Our experimental results of flow in the coiled capillary could not be explained by current models based on approximate solutions of the Navier–Stokes equation. We applied the technique to determine the diffusion coefficients of the following analytes: salts, drugs, single amino acids, peptides (from dipeptides to hexapeptides), and proteins.

We investigate a process of random walks of a point particle on a two-dimensional square lattice of size $n\times n$ with periodic boundary conditions. A fraction $p⩽20\%$ of the lattice is occupied by holes ($p$ represents macroporosity). A site not occupied by a hole is occupied by an obstacle. Upon a random step of the walker, a number of obstacles, $M$, can be pushed aside. The system approaches equilibrium in ${\left(n\ln n\right)}^2$ steps. We determine the distribution of $M$ pushed in a single move at equilibrium. The distribution $F\left(M\right)$ is given by $M^{\gamma }$ where $\gamma =-1.18$ for $p=0.1$, decreasing to $\gamma =-1.28$ for $p=0.01$. Irrespective of the initial distribution of holes on the lattice, the final equilibrium distribution of holes forms a fractal with fractal dimension changing from $a=1.56$ for $p=0.20$ to $a=1.42$ for $p=0.001$ (for $n=4,000$). The trace of a random walker forms a distribution with expected fractal dimension 2.

A detailed study of the reverse vesicles formed by a salt-free catanionic surfactant system in toluene was carried out by confocal fluorescence microscopy observations, dye-solubilizing tests, UV–vis measurements and cryo-TEM observations. When tetradecyltrimethyl ammonium laurate (TTAL) and lauric acid (LA) were mixed in the binary solution of water and toluene, reverse vesicular phase formed spontaneously. The reverse vesicular phase is quite stable and can be labeled with fluorescent dyes for subsequent confocal fluorescence microscopy observations. However, the reverse vesicles were found to have smaller sizes (<1 μm) and undergo structural evolutions in a much shorter time scale compared to those formed by the same surfactant mixture in cyclohexane, as shown in a previous report [19]. With extended observation time, interesting intermediate structures were observed including onions, sheets and cellular networks. The structural evolution pathways were deduced and the possibly influencing factors were discussed. Dye-solubilizing tests showed the ability of the reverse vesicles to accommodate dye molecules is smaller compared to those in cyclohexane. Besides, cryo-TEM observations were applied to probe the morphology of the reverse vesicles.

We performed synthesis and investigated the self-assembly properties of gold nanoparticles (NPs) with covalently attached bolaamphiphilic ligands (B-AuNPs). The judiciously designed coating rendered the NPs amphiphilic and induced their self-assembly. The B-AuNPs formed ordered two-dimensional structures over large areas upon simple drop-casting. The films exhibited an uncommon and applicable topography, consisting of densely packed rings of inner diameter of around 30 nm, with the B-AuNPs at the rim and an empty interior. We introduced and proved experimentally an explanation of how the structures were formed. The model involved elements of geometric packing and ligand reorganization. Upon contact with the hydrophilic surface, ligands rearranged at the surface of the metallic cores of the B-AuNPs so that the bolaamphiphilic moieties (constituting ca. 50% of the coating) were in proximity to the surface, while the hexanethiol moieties moved away from it. The described mechanism is of general relevance for the design of functional NPs capable of self-assembly.

We measure the activation energy $E_a$ for the diffusion of molecular probes (dyes and proteins of radii from 0.52 to 6.9 nm) and for macroscopic flow in a model complex liquid—aqueous solutions of polyethylene glycol. We cover a broad range of polymer molecular weights, concentrations, and temperatures. Fluorescence correlation spectroscopy and rheometry experiments reveal a relationship between the excess of the activation energy in polymer solutions over the one in pure solvent $\Delta E_a$ and simple parameters describing the structure of the system: probe radius, polymer hydrodynamic radius, and correlation length. $\Delta E_a$ varies by more than an order of magnitude in the investigated systems (in the range of ca. $1–15\mathrm{kJ}/\mathrm{mol}$) and for probes significantly larger than the polymer hydrodynamic radius approaches the value measured for macroscopic flow. We develop an explicit formula describing the smooth transition of $\Delta E_a$ from the diffusion of molecular probes to macroscopic flow. This formula is a reference for the quantitative analysis of specific interactions of moving nano-objects with their environment as well as active transport. For instance, the power developed by a molecular motor moving at constant velocity $u$ is proportional to $u^2\exp (E_a/RT)$.

Fluorescent double-stranded DNA (dsDNA) molecules labeled at both ends are commonly produced by annealing of complementary single-stranded DNA (ssDNA) molecules, labeled with fluorescent dyes at the same (3′ or 5′) end. Because the labeling efficiency of ssDNA is smaller than 100%, the resulting dsDNA have two, one or are without a dye. Existing methods are insufficient to measure the percentage of the doubly-labeled dsDNA component in the fluorescent DNA sample and it is even difficult to distinguish the doubly-labeled DNA component from the singly-labeled component. Accurate measurement of the percentage of such doubly labeled dsDNA component is a critical prerequisite for quantitative biochemical measurements, which has puzzled scientists for decades. We established a fluorescence correlation spectroscopy (FCS) system to measure the percentage of doubly labeled dsDNA (PDL) in the total fluorescent dsDNA pool. The method is based on comparative analysis of the given sample and a reference dsDNA sample prepared by adding certain amount of unlabeled ssDNA into the original ssDNA solution. From FCS autocorrelation functions, we obtain the number of fluorescent dsDNA molecules in the focal volume of the confocal microscope and PDL. We also calculate the labeling efficiency of ssDNA. The method requires minimal amount of material. The samples have the concentration of DNA in the nano-molar/L range and the volume of tens of microliters. We verify our method by using restriction enzyme Hind III to cleave the fluorescent dsDNA. The kinetics of the reaction depends strongly on PDL, a critical parameter for quantitative biochemical measurements.

Small changes in the alkane solvent structure in combination with temperature effects lead to four different conformations of stereoselectively deuterated benzene-1,3,5-tricarboxamides in the aggregated state, affecting the expression of the supramolecular chirality and highlighting the role of the solvent structure in self-assembly processes.

For two-dimensional lattices in a tight-binding description, the intrinsic spin-orbit coupling, acting as a complex next-nearest-neighbor hopping, opens gaps that exhibit the quantum spin Hall effect. In this paper, we study the effect of a real next-nearest-neighbor hopping term on the band structure of several Dirac systems. In our model, the spin is conserved, which allows us to analyze the spin Chern numbers. We show that in the Lieb, kagome, and $T_3$ lattices, variation of the amplitude of the real next-nearest-neighbor hopping term drives interesting topological phase transitions. These transitions may be experimentally realized in optical lattices under shaking, when the ratio between the nearest- and next-nearest-neighbor hopping parameters can be tuned to any possible value. Finally, we show that in the honeycomb lattice, next-nearest-neighbor hopping only drives topological phase transitions in the presence of a magnetic field, leading to the conjecture that these transitions can only occur in multigap systems.

A carbon nanotube/inorganic hybrid material has been fabricated by coupling Eu(III) complexes onto multi-walled carbon nanotubes (MWCNTs). Successful coupling has been verified by X-ray photoelectron spectroscopy (XPS) measurement where a clear signal from Eu3d has been identified. When sonicated in hexaethylene glycol monododecyl ether (C₁₂E₆) or sodium dodecyl sulfate (SDS) aqueous solutions, the MWCNTs with Eu-complex attached (denoted as Eu-MWCNTs hereafter) can be dispersed. UV–vis measurements on a dilute dispersion of Eu-MWCNTs in SDS aqueous solution reveal the characteristic absorption from Eu(III) complexes, which gives further proof of the successful coupling. The strong luminescent properties of Eu-MWCNTs allow them to be observed directly under a fluorescence microscope. Interestingly, it is found that Eu-MWCNTs can undergo continuous movements in C₁₂E₆ or SDS dilute solutions. When Eu-MWCNTs are incorporated into the lvotropic liquid crystal phase formed by C₁₂E₆ (above 40% by weight), however, movements have been hindered. Small angle X-ray scattering measurements showed that Eu-MWCNTs are ordered in the lyotropic liquid crystal.  Fluorescence microscopy observations reveal that the luminescent properties of the Eu-MWCNTs have not been affected by the liquid crystalline surfactant matrix.