Rayleigh−Bénard Instability in Sedimentation

One-Directional Fluidic Flow Induced by Chemical Wave Propagation in a Microchannel

A one-directional flow induced by chemical wave propagation was investigated to understand the origin of its dynamic flow. A cylindrical injection port was connected with a straight propagation channel; the chemical wave was initiated at the injection port. Chemical waves propagated with a constant velocity irrespective of the channel width, indicating that the dynamics of the chemical waves were governed by a geometry-independent interplay between the chemical reaction and diffusion. In contrast, the velocity of the one-directional flow was dependent on the channel width. Furthermore, enlargement of the injection port volume increased the flow velocity and volume flux. These results imply that the one-directional flow in the microchannel is due to a hydrodynamic effect induced in the injection port. Spectroscopic analysis of a pH indicator revealed the simultaneous behavior between the pH increase near the injection port and the one-directional flow. Hence, we can conclude that the one-directional flow in the microchannel with chemical wave propagation was caused by a proton consumption reaction in the injection port, probably through liquid volume expansion by the reaction products and the reaction heat. It is a characteristic feature of the present system that the hydrodynamic flow started from the chemical wave initiation point and not the propagation wavefront, as observed for previous systems.

Understanding the interfacial behavior in isopycnic Lennard-Jones mixtures by computer simulations

The physical characterization of the singular interfacial behavior of heterogeneous fluid systems is a very important step in preliminary stages of the design process, and also in the subsequent procedures for the determination of the optimal operating conditions. Molar isopycnicity or molar density inversion is a special case of phase equilibrium behavior that directly affects the relative position of phases in heterogeneous mixtures, without being affected by gravitational fields. This work is dedicated to characterize the impact of molar density inversion on the interfacial properties of Lennard-Jones binary mixtures. The results and specific trends of the molar density inversion phenomena on the peculiar calculated composition profiles across the interface and interfacial tensions are explored by using canonical molecular dynamics simulations of the Lennard-Jones binary mixtures. Our results show that the density inversion causes drastic changes in the density profiles of the mixtures. In particular, symmetrical and equal-sized Lennard-Jones mixtures always exhibit desorption along the interfacial zone, i.e. the interfacial concentration profiles show a relative minimum at the interface of the total density profiles that increases when the dispersive energy parameter (ε(ij)) between unlike species decreases. However, as the asymmetry of the Lennard-Jones mixtures increases (σ(i) ≠ σ(j)), the concentration profiles display a relative maximum at the interface, which implies the adsorption of the total density profiles along the interfacial zone.

High-speed, high-purity separation of gold nanoparticle-DNA origami constructs using centrifugation

DNA origami is a powerful platform for assembling gold nanoparticle constructs, an important class of nanostructure with numerous applications. Such constructs are assembled by the association of complementary DNA oligomers. These association reactions have yields of <100%, requiring the development of methods to purify the desired product. We study the performance of centrifugation as a separation approach by combining optical and hydrodynamic measurements and computations. We demonstrate that bench-top microcentrifugation is a simple and efficient method of separating the reaction products, readily achieving purities of >90%. The gold nanoparticles play a number of critical roles in our system, functioning not only as integral components of the purified products, but also as hydrodynamic separators and optical indicators of the reaction products during the purification process. We find that separation resolution is ultimately limited by the polydispersity in the mass of the gold nanoparticles and by structural distortions of DNA origami induced by the gold nanoparticles. Our study establishes a methodology for determining the design rules for nanomanufacturing DNA origami-nanoparticle constructs.

Density-gradient-free microfluidic centrifugation for analytical and preparative separation of nanoparticles

Sedimentation and centrifugation techniques are widely applied for the separation of biomolecules and colloids but require the presence of controlled density gradients for stable operation. Here we present an approach for separating nanoparticles in free solution without gradients. We use microfluidics to generate a convective flow perpendicular to the sedimentation direction. We show that the hydrodynamic Rayleigh-Taylor-like instability, which, in traditional methods, requires the presence of a density gradient, can be suppressed by the Poiseuille flow in the microchannel. We illustrate the power of this approach by demonstrating the separation of mixtures of particles on the nanometer scale, orders of magnitude smaller than the micrometer-sized objects separated by conventional inertial microfluidic approaches. This technique exhibits a series of favorable features including short analysis time, small sample volume, limited dilution of the analyte, limited interactions with surfaces as well as the possibility to tune easily the separation range by adjusting the geometry of the system. These features highlight the potential of gradient-free microfluidic centrifugation as an attractive route toward a broad range of nanoscale applications.

Speeding up of sedimentation under confinement

We show an increase of the sedimentation velocity as small particles are confined in circular capillaries. In general, confinement slows down sedimentation. But, we show that at low Reynolds numbers and in 1D confinement this is not the case. Particle sedimentation velocity is not homogeneous, which can lead to the formation of structures. These structures are enhanced and stabilized in the presence of walls and in the absence of other dissipative mechanisms. As a consequence, it is possible to achieve sedimentation velocities that even exceed the Stokes velocity. The segregation at critical capillary diameters has been directly observed using a large scale model. These simple experiments offer a new insight into the old problem of sedimentation under confinement.

Prolonging density gradient stability

For bottom-up particle fabrication, separation of complex particle assemblies from their precursor colloidal building blocks is critical to producing useable quantities of materials. The separations are often done using a density gradient sedimentation due to its simplicity and scalability. When loading density gradients at volume fractions greater than 0.001, however, an inherent convective instability arises. By translating the Rayleigh-Benard instability from the heat-transfer literature into an analogous mass-transfer problem, the variables affecting the critical stability limit were effectively catalogued and examined. Experiments using submicrometer particles loaded onto sucrose and Ficoll density gradients matched theoretical trends and led to a series of useful heuristics for prolonging density gradient stability. Higher particle loading heights, lower volume fractions, and smaller gradient material diffusion coefficients were found to improve stability. Centrifugation was useful at short times, as particles were removed from top of the gradient where the stable density profile degrades to unstable, and the resulting density inversion arises as the sucrose diffuses upward.