Self-propelling surfactant droplets in chemically-confined microfluidics – cargo transport, drop-splitting and trajectory control

We demonstrate the applicability of self-propulsion as a passive driving mechanism for droplets in chemically-confined microfluidics. The droplets can be used to transport considerably sized solid cargo particles. We implemented thermal actuation as a steering mechanism for the droplets at fluidic junctions

Insoluble surfactant spreading along thin liquid films confined by chemical surface patters

We conducted a combined experimental and numerical study of the spreading of insoluble surfactants on spatially confined thin liquid films. We found that the spreading dynamics can locally be represented by a power-law relation x B ta. We determine the time evolution of the liquid film thickness and the corresponding spreading exponents a both from experiments using interference microscopy and numerical finite element simulations. The lateral confinement induces non-uniform height- and surface velocity profiles, which manifest themselves in a pronounced transition of the evolving rivulet morphology. Excellent agreement between experimental and simulation results has been achieved

Selective dip-coating of chemically micropatterned surfaces

We characterize the selective deposition of liquid microstructures on chemically heterogeneous surfaces by means of dip coating processes. The maximum deposited film thickness depends critically on the speed of withdrawal as well as the pattern size, geometry, and angular orientation. For vertically oriented hydrophilic strips, we derive a hydrodynamic scaling relation for the deposited film thickness which agrees very well with interferometric measurements of dip-coated liquid lines. Due to the lateral confinement of the liquid, our scaling relation differs considerably from the classic Landau–Levich formula for chemically homogeneous surfaces. Dip coating is a simple method for creating large area arrays of liquid microstructures for applications involving chemical analysis and synthesis, biochemical assays, or wet printing of liquid polymer or ink patterns

Droplet actuation induced by coalescence: experimental evidences and phenomenological modeling

This paper considers the interaction between two droplets placed on a substrate in immediate vicinity. We show here that when the two droplets are of different fluids and especially when one of the droplet is highly volatile, a wealth of fascinating phenomena can be observed. In particular, the interaction may result in the actuation of the droplet system, i.e. its displacement over a finite length. In order to control this displacement, we consider droplets confined on a hydrophilic stripe created by plasma-treating a PDMS substrate. This controlled actuation opens up unexplored opportunities in the field of microfluidics. In order to explain the observed actuation phenomenon, we propose a simple phenomenological model based on Newton's second law and a simple balance between the driving force arising from surface energy gradients and the viscous resistive force. This simple model is able to reproduce qualitatively and quantitatively the observed droplet dynamics

Oblique roughness replication in strained SiGe/Si multilayers

The replication of the interface roughness in SiGe/Si multilayers grown on miscut Si(001) substrates has been studied by means of x-ray reflectivity reciprocal space mapping. The interface profiles were found to be highly correlated and the direction of the maximal replication was inclined with respect to the growth direction. This oblique replication is explained by the influence of the inhomogeneous strain distribution around step bunches. The formation of step bunches is described by a kinetic step-flow model based on the work by Tersoff et al. [Phys. Rev. Lett. 75, 2730 (1995)]. We have generalized this model by taking into account local variations of the in-plane strain. The angle of obliqueness deduced from these calculations agrees very well with the experimental findings

Oblique roughness replication in strained SiGe/Si multilayers

The replication of the interface roughness in SiGe/Si multilayers grown on miscut Si(001) substrates has been studied by means of x-ray reflectivity reciprocal space mapping. The interface profiles were found to be highly correlated and the direction of the maximal replication was inclined with respect to the growth direction. This oblique replication is explained by the influence of the inhomogeneous strain distribution around step bunches. The formation of step bunches is described by a kinetic step-flow model based on the work by Tersoff et al. [Phys. Rev. Lett. 75, 2730 (1995)]. We have generalized this model by taking into account local variations of the in-plane strain. The angle of obliqueness deduced from these calculations agrees very well with the experimental findings

Spin and energy transfer in nanocrystals without transport of charge

We describe a mechanism of spin transfer between individual quantum dots that does not require tunneling. Incident circularly-polarized photons create inter-band excitons with non-zero electron spin in the first quantum dot. When the quantum-dot pair is properly designed, this excitation can be transferred to the neighboring dot via the Coulomb interaction with either {\it conservation} or {\it flipping} of the electron spin. The second dot can radiate circularly-polarized photons at lower energy. Selection rules for spin transfer are determined by the resonant conditions and by the strong spin-orbit interaction in the valence band of nanocrystals. Coulomb-induced energy and spin transfer in pairs and chains of dots can become very efficient under resonant conditions. The electron can preserve its spin orientation even in randomly-oriented nanocrystals.Comment: 13 pages, 3 figure