177,059 research outputs found
Mobilities of a drop and an encapsulated squirmer
No full textAbstract We have analyzed the dynamics of a spherical, uniaxial squirmer which is located inside a spherical liquid drop at general position 1487911\varvec{r}_s1487911 r s . The squirmer is subject to an external force and torque in addition to the slip velocity on its surface. We have derived exact analytical expressions for the linear and rotational velocity of the squirmer as well as the linear velocity of the drop for general, non-axisymmetric configurations. The mobilities of both, squirmer and drop, are in general anisotropic, depending on the orientation of 1487911\varvec{r}_s1487911 r s , relative to squirmer axis, external force or torque. We discuss their dependence on the size of the squirmer, its distance from the center of the drop and the viscosities. Our results provide a framework for the discussion of the trajectories of the composite system of drop and enclosed squirmer. Graphical AbstractGeorg-August-Universität Göttinge
Self-propulsion of droplets driven by an active permeating gel
No full textWe discuss the flow field and propulsion velocity of active droplets, which are driven by body forces residing on a rigid gel. The latter is modelled as a porous medium which gives rise to permeation forces. In the simplest model, the Brinkman equation, the porous medium is characterised by a single lengthscale --the square root of the permeability. We compute the flow fields inside and outside of the droplet as well as the energy dissipation as a function of . We furthermore show that there are optimal gel fractions, giving rise to maximal linear and rotational velocities. In the limit , corresponding to a very dilute gel, we recover Stokes flow. The opposite limit, , corresponding to a space filling gel, is singular and not equivalent to Darcy’s equation, which cannot account for self-propulsion
Deformations of an active liquid droplet
Full text linkA fluid droplet in general deforms, if subject to active driving, such as a
finite slip velocity or active tractions on its interface. We show that these
deformations and their dynamics can be computed analytically in a perturbation
theory in the inverse surface tension using an approach based on vector
spherical harmonics. In lowest order, the deformation is of first order, yet it
affects the flow fields inside and outside of the droplet in zeroth order.
Hence a correct description of the flow has to allow for shape fluctuations,
even in the limit of large surface tension
Non-local linear stability of ion beam eroded surfaces
No full textContinuum theories of spontaneous pattern formation at solid surfaces during ion irradiation exist in many variants, but all of them are based upon low order gradient expansions of an underlying non-local theory and are formulated as partial differential equations. Here we reconsider the non-local theory based upon a simple Gaussian erosive crater function of Sigmund's theory of sputtering, which is also a basic ingredient of most of the existing continuum theories. We keep the full non-locality of the crater function in a linear stability analysis of a flat surface. Without gradient expansion the evolution of the height profile is governed by an integral equation. We show that low order gradient expansions may be misleading and that the bifurcation scenarios become significantly more complex, if the non-locality is taken into account. In a second step, we extend our analysis and include mass redistribution due to ion-induced drift currents of collision cascade atoms. The model is based upon results from kinetic theory and uses a simple phenomenology. Both erosion and mass redistribution share the same non-local features, as they are both caused by the collision cascade. If mass redistribution is the dominant pattern forming mechanism, we show that the resulting bifurcation scenarios may provide explanations for many of the recent, seemingly contradictory experimental results of pattern formation on Si surfaces. (C) 2011 Elsevier B. V. All rights reserved
Surfactant Sputtering: Theory of a new method of surface nanostructuring by ion beams
No full textWe present a new Monte Carlo model and a new continuum theory of surface pattern formation due to "surfactant sputtering", i.e. erosion by ion beam sputtering including a submonolayer coverage of additional, co-sputtered surfactant atoms. This setup, which has been realized in recent experiments in a controlled way leads to a number of interesting possibilities to modify pattern forming processing conditions. We will present three simple scenarios, which illustrate some potential applications of the method. In all three cases, simple Bradley-Harper type ripples appear in the absence of surfactant, whereas new, interesting structures emerge during surfactant sputtering. (C) 2009 Elsevier B.V. All rights reserved
From active stresses and forces to self-propulsion of droplets
No full textWe study the self-propulsion of spherical droplets as simplified hydrodynamic models of swimming micro-organisms or artificial micro-swimmers. In contrast to approaches that start from active velocity fields produced by the system, we consider active interface tractions, body force densities and active stresses as the origin of autonomous swimming. For negligible Reynolds number and given activity, we compute the external and internal flow fields as well as the centre of mass velocity and angular velocity of the droplet at fixed time. To construct trajectories from single time snapshots, the evolution of active forces or stresses must be determined in the laboratory frame. Here, we consider the case of active matter, which is carried by a continuously distributed rigid but sparse (cyto)-skeleton that is immersed in the droplet interior. We calculate examples of trajectories of a droplet and its skeleton from force densities or stresses, which may be explicitly time-dependent in a frame fixed within the skeleton.</jats:p
Controlled locomotion of a droplet propelled by an encapsulated squirmer
No full textAbstract
We work out the propulsion of a viscous drop which is driven by two mechanisms: the active velocity of an encapsulated squirmer and an externally applied force acting on the squirmer. Of particular interest is the existence of a stable comoving state of drop and squirmer, allowing for controlled manipulation of the viscous drop by external forcing. The velocities of droplet and squirmer, as well as the conditions for a stable comoving state are worked out analytically for the axisymmetric configuration with a general displacement of the squirmer from the center of the droplet
Graphic abstractProjekt DEA
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
