Lorentz Center - Physics of Micro- and Nanofluids from 9 Jun 2008 through 20 Jun 2008
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    Physics of Micro- and Nanofluids
    from 9 Jun 2008 through 20 Jun 2008






Phil Attard

The history of nanobubbles is reviewed, primarily focussing on the author's role in their discovery and characterization. The experimental evidence for them,
and the theoretical arguments against them, are discussed in detail. Possible explanations that reconcile the two are canvassed. Some consequences and applications of nanobubbles are mentioned. The long ranged hydrophobic attraction is also discussed in the context of nanobubbles and other possible explanations.


The Second Entropy: How Fast Time Flies
Phil Attard

The second entropy is introduced, which is a new type of entropy that is suitable for time-dependent systems [1]. A new law of thermodynamics is given that invokes the second entropy and that provides a basis for determining the rate of change of systems.  In the case of statistical mechanics, the probability distribution for an arbitrary non-equilibrium system is given. Quantitative evidence is provided that proves the validity of the theory. In particular, the non-equilibrium probability distribution is used to create the first ever non-equilibrium Monte Carlo algorithm. This is applied to the case of a Lennard-Jones fluid subject to an imposed thermal gradient, and the thermal conductivity is obtained and shown to be in agreement with known results. A Brownian particle subject to a time-varying external field is also analysed, and a stochastic molecular dynamics algorithm is given and shown to agree quantitatively with other approaches. The new theory is contrasted with the Principle of Maximum (or Minimum) Dissipation used  by Prigogine  and by others. The theory has interesting implications for evolution of life and for the rate of environmental change.

[1] P. Attard, Phys.  Chem.  Chem. Phys.   8, 3585 (2006).







The role of micromenisci at superhydrophobic surfaces

Frieder Mugele


The stability of the superhydrophobic state is determined by the properties of the liquid micromenisci spanning the gaps between adjacent peaks of the surface profile of the underlying solid. Ideally, the menisci have a constant mean curvature determined by the pressure in the liquid phase and three-phase contact lines are pinned to the ridges of the surface profile, in the present experiments the top edges posts arranged in a regular lattice. If the pressure in the liquid exceeds a certain threshold, the micromenisci become unstable and the superhydrophobic state collapses.

I will discuss a number of experiments in which we used a non-invasive optical diffraction and interference technique to determine the shape and the deflection amplitude of the liquid micromenisci as a function of a) the hydrostatic pressure in the liquid phase, b) acoustic fields impinging on the surface, and c) electrostatic fields in an electrowetting configuration.








Driven soft matter in nano-confinement

Roland Netz


A few exampes illustrate how soft matter under confinement reacts to external driving fields.

i) Surface-anchored polymers that are beating back and forth can be used to pump liquids over surfaces, which is a concept realized by biological ciliae but also attractive for synthetic designs. ii) Polymers in shear flows are repelled from surfaces, useful for desorption of DNA in gene-chip applications.

iii) Structured surfaces consisting of obstacles can be used for efficient

length-dependent sieving

iv) The dynamic charging of nano capacitors filled with liquid electrolyte

shows strong departure from the mean-field type Poisson-Nernst-Planck equation, which is traced back to various types of dynamic structure formation.









Wetting Failure and contact line dynamics in a Couette Flow
M. Sbragaglia

Liquid-liquid wetting failure is here investigated for the case of contact  line motion in a two-dimensional Couette system with two immiscible fluids
of arbitrary  viscosity.  The problem is solved exactly using a  sharp interface treatment of hydrodynamics (lubrication theory) as a function of the capillary number, viscous ratio, and separation of  scale, i.e. slip length  versus macroscopic scale of the system.  The existence of critical velocities, above which no stationary solutions are found, will be analyzed in detail in terms of the relevant parameters of the system. Comparisons with existing analysis for other geometries  are also carried out. Finally, another computational method of analysis  is  presented, based on diffuse interface models obtained from multiphase extensions of the Lattice Boltzmann equation (LBE). Sharp interface and diffuse interface models will be quantitatively compared face to face indicating the correct limit of applicability of the diffuse interface models.