Particle accumulation and depletion in a microfluidic Marangoni flow.

Thermosolutal and thermocapillary Marangoni convection at a liquid-gas interface in a microchannel structure of approximately 100 × 90 µm2 cross section creates a localized vortex that acts as a trap for micrometer and sub-micrometer sized tracer particles. Next to the vortex, depleted volumes appear that are entirely cleared of particles. This particle redistribution is caused by collisions of the tracers with the meniscus, which push the particles back onto the critical streamline with one particle radius distance to the meniscus. The streamlines between the meniscus and the critical streamline feed the depleted regions. Since the critical streamline depends on the particle radius, the effect leads to a particle fractionation according to their size. Diffusion allows only small particles to escape from the trap. Larger particles are permanently confined and their diffusion is rectified after every revolution at the meniscus, which produces a ratchet effect and increases the particle localization within the vortex.

Competition between phase transition and thermophoretic expansion of a transient poly(N-isopropylacrylamide) network.

Aqueous solutions of highly entangled ultra-high molar mass ( 2.4×106 g/mol) poly(N-isopropylacrylamide) (PNIPAM) have been subjected to an inhomogeneous temperature field by selective heating of a single embedded gold nanoparticle (GNP) by means of a focused laser beam. Randomly distributed tracer GNPs are trapped in the meshes of the transient entanglement network and serve as tracers for the monitoring of the network deformation field. Because of the positive Soret coefficient of PNIPAM in water, the viscoelastic polymer network is expanded by thermophoretic forces pointing away from the hot center. Close to the heated GNP the thermoresponsive polymer solution crosses the binodal and the network contracts, which is made visible by an inward motion of tracer GNPs, which are randomly embedded in the polymer network and not illuminated by the laser beam. Within a thin transition zone the network contraction and the competing thermophoretic network expansion cancel out in the steady state. There is, however, no cancellation during the transients due to the different time scales of both mechanisms. The network within the crossover region first undergoes an expansion that is followed by a slower contraction. From the global expansion and contraction, the local strain (stretching and compression) of the transient network can be calculated. Due to the long disentanglement times, corresponding to long lifetimes of the meshes of the network, the whole process is fully reversible.

Forced Phase Separation by Laser-Heated Gold Nanoparticles in Thermoresponsive Aqueous PNIPAM Polymer Solutions.

We have investigated the formation, growth, and dissolution dynamics of aggregates of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM) that form around laser heated gold nanoparticles (GNPs). The aggregates show an initial rapid growth followed by a slow long-term tail that is caused by the temperature dependent induction time until phase separation sets in. The maximum aggregate radius is determined by the distance from the GNP where the temperature crosses the binodal. Melting and evaporation of the GNP can be identified as characteristic steps in the aggregate size as a function of the heating laser power. After prolonged exposure, the polymer concentration inside the aggregate increases considerably. GNPs get immobilized at the perimeter, and a stepwise increase of the laser power results in onionskin-like growth shells. After switching the laser off, the system returns to the homogeneous phase and the growth shells are radially repelled by the high osmotic pressure within the volume previously occupied by the aggregate.