Drying of colloidal solutions of fluoroalkyl oligomers

The change in the supramolecular structure upon drying (solvent removal) of colloidal solutions of fluoroalkyl oligomers at atmospheric pressure has been studied using atomic force microscopy. In an initial colloidal solution, micrometer-sized particles of the dense phase consist of randomly oriented oligomers in the form of rigid rods of a 3–5 nm length forming a porous framework filled with solvent molecules, which solvate the oligomer chains. The drying-induced capillary pressure, which in nanosized pores is of the same order of magnitude as the solvation energy, leads to framework deformation, collapse of the pores, and the formation of lamellar and dendritic structures on a 50–100 nm scale. The ordering of these structures (formation of blocks of parallel oriented fibers typical of a fluoroplastic) increases as the heat-treatment temperature and the drying rate are increased, increasing the roughness of the surface (ratio of real to smooth surface area) and its hydrophobicity.     

Visualization of supramolecular framework of perfluorinated-oligomer colloid particles by supercritical drying

Aerogels of the tetrafluoroethylene radical polymerization products H(C₂F₄)_nR, where R is the radical formed by the abstraction of a hydrogen atom from a solvent molecule, have been obtained by replacing the solvent with supercritical CO₂ and its subsequent rapid evaporation. According to the data of scanning electron and atomic force microscopy, the aerogel consists of loosely bound particles of 1–3 μm in diameter, which is two to three times that of colloid particles in the initial solution, where the particles consist of an oligomer framework filled with solvent molecules. The internal structure of the framework is manifested in the surface topography with a roughness coefficient of 1.6–1.8. High roughness leads to the formation of ultrahydrophobic coatings with contact angles of >160°. A model of supercritical drying in which the solvent is removed from the colloidal particles without alteration of the supramolecular structure is discussed.     

Dissipative tunneling in nanosystems

A quantum dynamical problem has been analytically solved for a two-level system where localized states L ₀ and R ₀ are strongly coupled with reservoirs of local oscillations {L _n } and {R _n }. It is additionally assumed that the spectra of reservoirs are equidistant and the coupling constants are the same. It has been shown that the evolution of states L ₀ and R ₀ in recurrence cycles depends on three independent factors, which characterize exchange with the two-level system, exchange of L ₀ with {L _n } (R ₀ with {R _n }) and the phonon-induced decay of {L _n } and {R _n }. In addition to coherent oscillations with the frequency of the two-level system, Δ, and dissipative tunneling with a rate Δ²/πC ² (where C is the matrix element of the coupling of L ₀ and R ₀ with L _n and R _n ), a new regime appears where L-R transitions are induced by the partial recovery of the populations of L ₀ and R ₀ in each recurrence cycle due to synchronous transitions from reservoirs. These transitions induce repeating changes in the populations of the states of the two-level system (Loschmidt echo). The number and width of the echo components increase with the cycle number. Evolution becomes irregular because of the mixing of the contributions from pulses of the neighboring cycles, when the cycle number k exceeds the critical value k _c = π² C ². Unlike the populations, their cycle-average values remain regular at k ≫ k _c. When Δ ≪ πC ², the cycle-average populations oscillate with a frequency of ΔΩ/πC ² irrespective of mixing. The frequency of oscillations of the populations of the states {L _n } and {R _n } is approximately nΩ(Δ/2πC ²)², where Ω is the spacing between the neighboring levels of the reservoir and nΩ is the difference between the energies of the states L ₀ and L _n . The appearance of the mentioned low-frequency oscillations is due to the formation of collective states of the two-level system that are “dressed” by the reservoir. The predicted oscillations can be detected by femtosecond spectroscopy methods.     

Analysis of Noise Generation by Turbulent Jets from Consideration of Their Near Acoustic Field

Acoustical Physics - The paper studies the generation of acoustic waves near the boundaries of swirling and nonswirling turbulent jets outside a jet flow. Nonstationary motion of the medium is...     

Propagation of heat in anthracene crystals at helium temperatures

Measurement of temperature pulses with time resolution less than 5 nsec based, on the high-speed registration of the Debye-Waller factor in the fluourescence spectrum of an impurity layer deposited on a sample, has been achieved. In anthracene single crystals, the velocity of heat propagation was found to be equal to 0.7?1.0.10⁵ cm/sec, approaching that sound; the mean free path of phonons was found to exceed 30 μm.     

New mechanism for non-trivial intra-molecular vibrational dynamics

We investigate the time evolution process of one selected (initially prepared by optical pumping) vibrational molecular state S, coupled to all other intra-molecular vibrational states R of the same molecule, and also to its environment Q. Molecular states forming the first reservoir R are characterized by a discrete dense spectrum, whereas the environment reservoir Q states form a continuous spectrum. Assuming the equidistant reservoir R states we find the exact analytical solution of the quantum dynamic equations. S-Q and R-Q couplings yield to spontaneous decay of the S and R states, whereas S-R exchange leads to recurrence cycles and Loschmidt echo at frequencies of S-R transitions and double resonances at the interlevel reservoir R transitions. Due to these couplings the system S time evolution is not reduced to a simple exponential relaxation. We predict various regimes of the system S dynamics, ranging from exponential decay to irregular damped oscillations. Namely, we show that there are possible four dynamic regimes of the evolution: (i) independent of the environment Q exponential decay suppressing backward R - S transitions, (ii) Loschmidt echo regime, (iii) incoherent dynamics with multicomponent Loschmidt echo, when the system state is exchanged its energy with many states of the reservoir, (iv) cycle mixing regime, when long time system dynamics looks as a random-like. We suggest applications of our results for interpretation of femtosecond vibration spectra of large molecules and nano-systems.     

Vibrational energy transport in molecular wires

Motivated by recent experimental observation (see, e.g., I. V. Rubtsov, Acc. Chem. Res. 42, 1385 (2009)) of vibrational energy transport in (CH₂O) _N and (CF₂) _N molecular chains (N = 4–12), in this paper we present and solve analytically a simple one dimensional model to describe theoretically these data. To mimic multiple conformations of the molecular chains, our model includes random off-diagonal couplings between neigh-boring sites. For the sake of simplicity, we assume Gaussian distribution with dispersion σ for these coupling matrix elements. Within the model we find that initially locally excited vibrational state can propagate along the chain. However, the propagation is neither ballistic nor diffusion like. The time T _m for the first passage of the excitation along the chain, scales linearly with N in the agreement with the experimental data. Distribution of the excitation energies over the chain fragments (sites in the model) remains random, and the vibrational energy, transported to the chain end at t = T _m is dramatically decreased when σ is larger than characteristic interlevel spacing in the chain vibrational spectrum. We do believe that the problem we have solved is not only of intellectual interest (or to rationalize mentioned above experimental data) but also of relevance to design optimal molecular wires providing fast energy transport in various chemical and biological reactions.