Shape-Based Separation of Micro-/Nanoparticles in Liquid Phases

The production of particles with shape-specific properties is reliant upon the separation of micro-/nanoparticles of particular shapes from particle mixtures of similar volumes. However, compared to a large number of size-based particle separation methods, shape-based separation methods have not been adequately explored. We review various up-to-date approaches to shape-based separation of rigid micro-/nanoparticles in liquid phases including size exclusion chromatography, field flow fractionation, deterministic lateral displacement, inertial focusing, electrophoresis, magnetophoresis, self-assembly precipitation, and centrifugation. We discuss separation mechanisms by classifying them as either changes in surface interactions or extensions of size-based separation. The latter includes geometric restrictions and shape-dependent transport properties

Numerical Study on the Temperature-Dependent Viscosity Effect on the Strand Shape in Extrusion-Based Additive Manufacturing

A numerical model that incorporates temperature-dependent non-Newtonian viscosity was developed to simulate the extrusion process in extrusion-based additive manufacturing. Agreement with the experimental data was achieved by simulating a polylactic acid melt flow as a non-isothermal power law fluid using experimentally fitted parameters for polylactic acid. The model was used to investigate the temperature effect on the flow behavior, the cross-sectional area, and the uniformity of the extruded strand. OpenFOAM, an open source simulation tool based on the finite volume method, was used to perform the simulations. A computational module for solving the equations of non-isothermal multiphase flows was also developed to simulate the extrusion process under a small gap condition where the gap between the nozzle and the substrate surface is smaller than the nozzle diameter. Comparison of the strand shapes obtained from our model with isothermal Newtonian simulation, and experimental data confirms that our model improves the agreement with the experimental data. The result shows that the cross-sectional area of the extruded strand is sensitive to the temperature-dependent viscosity, especially in the small gap condition which has recently increased in popularity. Our numerical investigation was able to show nozzle temperature effects on the strand shape and surface topography which previously had been investigated and observed empirically only

Engineering Polarizability of Janus Particles to Achieve Efficient Self Propulsion

Inspired by swarming motion of living organisms in nature, active particles use the stored energy in the environment to initiate self-propulsion. Successful use of active particles in different applications, including cargo transport and drug delivery, relies on their capacity to travel in predefined directions. Despite recent advances, role of polarizability in effective steering of particles under AC field has not been investigated. In this dissertation, the goal is to understand how polarizability manifests itself in the self-propulsion of particles. Metallodielectric Janus particles (JPs) serve as an effective model system for investigating active motion and collective dynamics under AC fields. Particles were fabricated by depositing sequential layers of titanium and silica on a dielectric (PS or Silica) core. Electrorotation and electroorientation measurements, along with a Poisson–Boltzmann–Nernst–Planck model in COMSOL Multiphysics, revealed two characteristic relaxation times for the polarization of JPs. At lower frequencies (below 10kHz), relaxation is attributed to the charging of an induced double layer at the particle–electrolyte interface, whereas at higher frequencies (~1MHz), the relaxation is due to the classical Maxwell–Wagner interfacial polarization. We observed that the spectra for propulsion velocity exhibited features (amplitude and transition frequencies) that closely aligned with the polarizability spectra. The insights gained from our numerical simulations prompt an expansion of our research into multiphase systems where we explore selective dynamics occurring around the air/water interface. This system allows for low-frequency attainment of rotational, translational, and interplanar motions by adjusting particle concentration and external field frequency, while at high frequencies, the particles undergo assembly at the interface. Findings in this dissertation provide a framework for designing colloids with targeted dynamical properties for the transport of cargo or microrobots. Moreover, the results can help link the complex emerging collective behavior driven by electric fields to the properties of individual particles

Visualization of Concentration Gradients and Colloidal Dynamics under Electrodiffusiophoresis

In this work, we present an experimental study of the dynamics of charged colloids under direct currents and gradients of chemical species (electrodiffusiophoresis). In our approach, we simultaneously visualize the development of concentration polarization and the ensuing dynamics of charged colloids near electrodes. With the aid of confocal microscopy and fluorescent probes, we show that the passage of current through water confined between electrodes, separated about a hundred microns, results in significant pH gradients. Depending on the current density and initial conditions, steep pH gradients develop, thus becoming a significant factor in the behavior of charged colloids. Furthermore, we show that steep pH gradients induce the focusing of charged colloids away from both electrodes. Our results provide the experimental basis for further development of models of electrodiffusiophoresis and the design of non-equilibrium strategies for materials fabrication

Electric polarizability of metallodielectric Janus particles in electrolyte solutions

Experiments and transport analysis describe the electric polarizability of JPs in a wide range of frequencies.</jats:p

Long-range transport and directed assembly of charged colloids under aperiodic electrodiffusiophoresis

AC faradaic reactions promote focusing and crystallization away from electrodes under EDP.</jats:p

Direct Numerical Simulation of Microbubble Streaming in a Microfluidic Device: The Effect of the Bubble Protrusion Depth on the Vortex Pattern

Microbubble streaming in a microfluidic device has been increasingly studied and used in recent years, due to its unique flow pattern that can promote mixing, sort particles and trap particles in microscale flows. However, there have been few numerical studies of this subject. We performed a 3D direct simulation of a cylindrical-shaped micro-bubble, trapped in a pit of a microchannel and sandwiched between two parallel plates, vibrated by pressure oscillation. Our simulation was able to reproduce the experimentally observed relation between the bubble protrusion depth and the vortex pattern: As the bubble protrusion depth increased, new vortices emerged and grew larger. Our investigation of the streamlines near the bubble interface indicates that the number of non-spherical nodes in the bubble interface is closely related to the flow pattern in the liquid phase. It was also validated by our simulation that the flow velocity showed an exponentially decaying trend as the radial distance outward from the vortex center. Our numerical model was also used to investigate the effects of surface tension and channel size on the vortex pattern. Larger surface tension or smaller channel size showed a similar effect as the increased protrusion depth induced more vortices