Self-propelled Leidenfrost droplets

We report that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchetlike) topology. The pumping effect is observed when the liquid is in the Leidenfrost regime (the film-boiling regime), for many liquids and over a wide temperature range. We propose that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid.     

Self-propelled running droplets on solid substrates driven by chemical reactions

We study chemically driven running droplets on a partially wetting solid substrate by means of coupled evolution equations for the thickness profile of the droplets and the density profile of an adsorbate layer. Two models are introduced corresponding to two qualitatively different types of experiments described in the literature. In both cases an adsorption or desorption reaction underneath the droplets induces a wettability gradient on the substrate and provides the driving force for droplet motion. The difference lies in the behavior of the substrate behind the droplet. In case I the substrate is irreversibly changed whereas in case II it recovers allowing for a periodic droplet movement (as long as the overall system stays far away from equilibrium). Both models allow for a non-saturated and a saturated regime of droplet movement depending on the ratio of the viscous and reactive time scales. In contrast to model I, model II allows for sitting drops at high reaction rate and zero diffusion along the substrate. The transition from running to sitting drops in model II occurs via a super- or subcritical drift-pitchfork bifurcation and may be strongly hysteretic implying a coexistence region of running and sitting drops.     

Self-propelled particle model for cell-sorting phenomena

A self-propelled particle model is introduced to study cell sorting occurring in some living organisms. This allows us to evaluate the influence of intrinsic cell motility separately from differential adhesion with fluctuations, a mechanism previously shown to be sufficient to explain a variety of cell rearrangement processes. We find that the tendency of cells to actively follow their neighbors greatly reduces segregation time scales. A finite-size analysis of the sorting process reveals clear algebraic growth laws as in physical phase-ordering processes, albeit with unusual scaling exponents.     

Dilute quantum droplets

In the limit when the two-body scattering length a is negative and much larger than the effective two-body interaction radius the contribution to the ground state energy due to the three-body correlations is given by the Efimov effect. For particular values of the diluteness parameter rho/a/(3) the three-body contribution can become the dominant term of the energy density functional. Under these conditions both Bose-Einstein and Fermi-Dirac systems could become self-bound and either boson droplets or fermion "designer nuclei" of various sizes and densities could be manufactured.     

Ferromagnetic liquid droplets

Experimental evidence for ferromagnetic behavior of liquid droplets produced by laser ablation from amorphous alloys is presented for the first time. Thin films of amorphous magnetic materials are fabricated by a laser deposition technique in the presence and in the absence of magnetic field. The differences in the parameters of deposited films are attributed to the ferromagnetic properties of small liquid droplets. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 686–689 (10 May 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Dynamics of non-Newtonian droplets

We study the impact and subsequent retraction dynamics of aqueous liquid droplets upon high-speed impact on hydrophobic surfaces. Often a spectacular "rebound" of the droplet can be observed: after the impact and expansion, the drop retracts rapidly, leading to ejection of part of the material from the surface. We show how non-Newtonian flow properties can be used to slow down the retraction sufficiently to completely inhibit rebound. The slowing down is due to non-Newtonian normal stresses generated near the moving contact line of the droplet. We provide a quantitative theory for the slowing down, and show that the non-Newtonian effects profoundly change the contact line dynamics.     

High-resolution mass spectrometric study of pure helium droplets,and droplets doped with krypton

Mass spectra of doped and undoped helium droplets are presented. The high resolution of the time-of-flight spectrometer (m/Δm ≅ 5000) makes it possible to fully resolve small helium cluster ions from impurities and to unambiguously identify abundance anomalies in the size distribution of He _n ⁺. The yield of He₄ ⁺ shows the well-known enhancement relative to other small cluster ions when the expansion changes from sub- to supercritical, provided the electron energy exceeds a value of 40 ± 1 eV, the threshold for formation of electronically excited ions. Upon doping with krypton, pure Kr _n ⁺ cluster ions containing up to 41 Kr atoms are observed. The spectra exhibit abundance anomalies at 13, 16, 19, 22 & 23, 26 and 29, in agreement with spectra obtained by ionization of bare krypton clusters that are formed in neat supersonic beams. Mixed clusters He _m Kr⁺ indicate closure of a solvation shell at m = 12.     

Electron — positron quantum droplets

A new physical object, electron-positron quantum droplet, is suggested. Structure, stability and dynamics of such objects are discussed. The analysis is based on the non-relativistic self-consistent local-density approximation. An essential role of many-body effects in the formation of the droplets is demonstrated. Their properties are compared with the known physical objects such as metal clusters and clusters of excitons in a solid.     

Crystallization of solutes from droplets

Experiments on crystallization of thiourea and sodium hydroxide from an aqueous solution in a microvolume of 10⁻¹⁰ 1 (100 pl) are described. The phenomenon of dehydration self-organization in such tiny droplets of solution is studied for the first time. The effect of different factors, such as solution concentration, ratio of the components mentioned above in one solution, and substrate properties on the size, structure, and properties of crystals grown. Analysis of the experimental data is performed, and the drying rate of the drops and the supersaturation of the solution are estimated. Insight into the self-organization of solute molecules in a micro-or nanovolume of the drying solution may be of great applied and fundamental value, giving rise to new techniques in micro-and nanotechnology and new classes of problems in the physics of fractals and strongly nonstationary dissipative processes.     

Electron-hole droplets size at equilibrium

A finite lifetime τ implies that the liquid phase can only form droplets of finite size. Introducing this finite τ in the usual nucleation theory, one can obtain analytical expressions for the electron-hole droplets size, number and exciton density at steady state, in the limit of large τ. This equilibrium size results, in fact, from a Maxwell construction on an appropriate chemical potential.     

从下落的水滴中收集能量

两个世纪前,考古学家在古巴比伦废墟中的挖掘,得到了一个有趣的发现——追溯到公元前约1800年,用楔形文字书写的粘土碑。这些碑文包括对迷信行为的描述,神职人员通过倾倒在水上的油形成的图样,预言未来。这篇古文代表了已知最早的关于疏水效应的参考文献。现在两个独立的现代科学家团队已经使用这个想法,开发了非常有效的没有移动部件的纳米发电机。使纳米发电机有效的关键技术是利用疏水效应。     

Heat transfer within combusting droplets

The improvement of the basic understanding of heat transfer in sprays is a key point in many engineering applications. In this paper, the temperature field within combusting ethanol droplets in linear stream is investigated by the two-color laser induced fluorescence technique. Additionally, a heat transfer model within the droplet is developed, taking into account both heat conduction and heat advection by the droplet internal fluid circulation, according to the Hill vortex pattern. Heat and mass exchanges between the liquid and the gas phases are described within the framework of the quasi-steady approach and the film theory. Comparisons between measurements and computational results allow determining the intensity of the Hill vortex related to the maximum velocity at the droplet surface. An expression of the friction coefficient for combusting and interacting droplets is derived from the case of an isolated droplet and a good agreement with the experimental data is observed.     

Ising droplets in five dimensions

Monte Carlo simulations indicate that the size distribution of Coniglio-Klien droplets in the five-dimensional nearest-neighbor Ising model corresponds to mean field Ising exponents.     

Decompressing emulsion droplets favors coalescence

The destabilization process of an emulsion under flow is investigated in a microfluidic device. The experimental approach enables us to generate a periodic train of droplet pairs, and thus to isolate and analyze the basic step of the destabilization, namely, the coalescence of two droplets which collide. We demonstrate a counterintuitive phenomenon: coalescence occurs during the separation phase and not during the impact. Separation induces the formation of two facing nipples in the contact area that hastens the connection of the interfaces prior to fusion. Moreover, droplet pairs initially stabilized by surfactants can be destabilized by forcing the separation. Finally, we note that the fusion mechanism is responsible for a cascade of coalescence events in a compact system of droplets where the separation is driven by surface tension.