Sai Nudurupati

Micro- and nanoparticles self-assembly for virtually defect-free, adjustable monolayers

As chips further shrink toward smaller scales, fabrication processes based on the self-assembly of individual particles into patterns or structures are often sought. One of the most popular techniques for two-dimensional assembly (self-assembled monolayers) is based on capillary forces acting on particles placed at a liquid interface. Capillarity-induced clustering, however, has several limitations: it applies to relatively large (radius > ≈10 μm) particles only, the clustering is usually non-defect-free and lacks long-range order, and the lattice spacing cannot be adjusted. The goal of the present article is to show that these shortcomings can be addressed by using an external electric field normal to the interface. The resulting self-assembly is capable of controlling the lattice spacing statically or dynamically, forming virtually defect-free monolayers, and manipulating a broad range of particle sizes and types including nanoparticles and electrically neutral particles.

Concentrating particles on drop surfaces using external electric fields

We propose to use an externally applied uniform electric field to alter the distribution of particles on the surface of a drop immersed in another immiscible liquid. Specifically, we seek to generate well‐defined concentrated regions at the drop surface while leaving the rest of the surface particle free. Experiments show that when the dielectric constant of the drop is greater than that of the ambient liquid the particles for which the Clausius–Mossotti factor is positive move along the drop surface to the two poles of the drop. Particles with a negative Clausius–Mossotti factor, on the other hand, move along the drop surface to form a ring near the drop equator. The opposite takes place when the dielectric constant of the drop is smaller than that of the ambient liquid, namely particles for which the Clausius–Mossotti factor is positive form a ring near the equator while those for which such a factor is negative move to the poles. This motion is due to the dielectrophoretic force that acts upon particles because the electric field on the surface of the drop is nonuniform, despite the uniformity of the applied electric field. Experiments also show that when small particles collect at the poles of a deformed drop the electric field needed to break the drop is smaller than without particles. These phenomena could be useful to concentrate particles at a drop surface within well‐defined regions (poles and equator), separate two types of particles at the surface of a drop or increase the drop deformation to accelerate drop breakup.

Electric field‐induced self‐assembly of micro‐ and nanoparticles of various shapes at two‐fluid interfaces

Particle lithography which explores the capability of particles to self‐assemble offers an attractive means to manufacture nanostructured materials. Although traditional techniques typically lead to the formation of dense crystals, adjustable non‐close‐packed crystals are crucial in a number of applications. We have recently proposed a novel method to assemble spherical micro‐ and nanoparticles into monolayers. The technique consists of trapping particles at a liquid–fluid interface and applying an electric field normal to the interface. Particles rearrange themselves under the influence of interfacial and electrostatic forces to form 2‐D hexagonal arrays of long‐range order and whose lattice constant depends on the electric field strength and frequency. Furthermore, the existence of an electric field‐induced capillary force makes the technique applicable to submicron and nanosized particles. Although spherical particles are often used, non‐spherical particles can be beneficial in practice. Here, we review the method, discuss its applicability to particles of various shapes, and present results for particles self‐assembly on air–liquid and liquid–liquid interfaces. In the case of non‐spherical particles, the self‐assembly process, while still taking place, is more complex as particles experience a torque which causes them to rotate relative to one another. This leads to a final arrangement displaying either a dominant orientation or no well‐defined orientation. We also discuss the possibility of dislodging the particles from the interface by applying a strong electric field such that the Weber number is of order 1 or larger, a phenomenon which can be utilized to clean particles from liquid–fluid surfaces.

Spontaneous dispersion of particles on liquid surfaces

When small particles (e.g., flour, pollen, etc.) come in contact with a liquid surface, they immediately disperse. The dispersion can occur so quickly that it appears explosive, especially for small particles on the surface of mobile liquids like water. This explosive dispersion is the consequence of capillary force pulling particles into the interface causing them to accelerate to a relatively large velocity. The maximum velocity increases with decreasing particle size; for nanometer-sized particles (e.g., viruses and proteins), the velocity on an air-water interface can be as large as ≈47 m/s. We also show that particles oscillate at a relatively high frequency about their floating equilibrium before coming to stop under viscous drag. The observed dispersion is a result of strong repulsive hydrodynamic forces that arise because of these oscillations.

Electrohydrodynamics of yeast cells in microchannels subjected to travelling electric fields

While travelling wave dielectrophoresis (twDEP) offers a promising method for the control of micro-sized particles suspended in liquids, particularly when the motion of particles along the length of the channel is sought without having to pump the liquid itself, it leads to a variety of complex dynamical regimes which need to be clearly understood for the design of efficient microfluidic devices targeting particular functions. In this paper, we describe the various dynamical regimes in terms of the forces acting on the particles, i.e. the conventional dielectrophoretic and twDEP force and torque, the viscous drag exerted by the fluid on the particle and the electrostatic and hydrodynamic particle–particle interactions. We also explore the variation of the dynamical regimes in three different configurations typical of microfluidic channels whose electrodes are embedded in the bottom wall. The first two configurations have different, i.e. aligned and staggered, electrode geometries, and the third configuration consists of aligned electrodes but energized at different potentials. For these purposes, we use our direct numerical simulation code based on the distributed Lagrange multiplier method for solving the equations of motion for both the fluid and the individual particles, and the point dipole model to compute the electrostatic forces. The model particles are chosen so as to have mechanical and electrical properties of yeast cells suspended in an aqueous solution. It is found that the motion of the particles not only depends significantly on the Clausius–Mossotti factor, which is a function of the electric properties of the fluid and the particles, but also on the specific configuration considered. Particularly, the spinning of particles plays a crucial role in the particle translations and interactions, but the direction of such spinning motion depends on the particular device configuration used.

Redistribution and Removal of Particles From Drop Surfaces

It was recently shown by us that particles distributed on the surface of a drop can be concentrated at the poles or equator of the drop by subjecting the latter to a uniform electric field. In this paper, we present experimental results for the dependence of the dielectrophoretic force on the parameters of the system such as the particles’ and drop’s radii and the dielectric properties of the fluids and particles, and define a dimensionless parameter regime for which the technique can work. Specifically, we show that if the drop radius is larger than a critical value, that depends on the physical properties of the drop and ambient fluids and the particles, it is not possible to concentrate particles and thus clean the drop of the particles it carries at its surface because the drop breaks or tip-streams at an electric field intensity smaller than that needed for concentrating particles. However, since the dielectrophoretic force varies inversely with the drop radius, the effectiveness of the concentration mechanism increases with decreasing drop size, and therefore the technique is guaranteed to work provided the drop radius is sufficiently small.Copyright

Modeling of Particles Dispersion on Liquid Surfaces

When small particles (e.g., flour, pollen, etc.) come in contact with a liquid surface, they immediately disperse. The dispersion can occur so quickly that it appears explosive, especially for small particles on the surface of mobile liquids like water. This explosive-like dispersion is the consequence of capillary forces pulling particles into the interface causing them to accelerate to a relatively large velocity. The maximum velocity increases with decreasing particle size; for nanometer-sized particles (e.g., viruses and proteins), the velocity on an air-water interface can be as large as 47 m/s. We also show that particles oscillate at a relatively-high frequency about their floating equilibrium before coming to stop under viscous drag. The observed dispersion is a result of strong repulsive hydrodynamic forces that arise because of these oscillations.Copyright

Self-Assembly of Rod-Like Particles Into 2D Lattices

It was recently shown by us that spherical particles floating on a fluid-fluid interface can be self-assembled, and the lattice between them can be controlled, using an electric field. In this paper we show that the technique can also be used to self assemble rod-like particles on fluid-fluid interfaces. The method consists of sprinkling particles at a liquid interface and applying an electric field normal to the interface, thus resulting in a combination of hydrodynamic (capillary) and electrostatic forces acting on the particles. A rod floating on the fluid interface experiences both a lateral force and a torque normal to the interface due to capillarity, and in the presence of an electric field, it is also subjected to an electric force and torque. The electric force affects the rods’ approach velocity and the torque aligns the rods parallel to each other. In the absence of an electric field, two rods that are initially more than one rod length away from each other come in contact so that they are either perpendicular or parallel to the line joining their centers, depending on their initial orientations. In the latter case, their ends are touching. Our experiments show that in an electric field of sufficiently large strength, only the latter arrangement is stable. Experiments also show that in this case the electric field causes the rods of the monolayer to align parallel to one another and that the lattice spacing of a self-assembled monolayer of rods increases.© 2008 ASME

Removal of Particles From the Surface of a Droplet

We present a technique to concentrate particles on the surface of a drop, separate different types of particles, and remove them from the drop by subjecting the drop to a uniform electric field. The particles are moved under the action of the dielectrophoretic force which arises due to the non-uniformity of the electric field on the surface of the drop. Experiments show that depending on the dielectric constants of the fluids and the particles, particles aggregate either near the poles or near the equator of the drop. When particles aggregate near the poles and the dielectric constant of the drop is greater than that of the ambient fluid, the drop deformation is larger than that of a clean drop. In this case, under a sufficiently strong electric field the drop develops conical ends and particles concentrated at the poles eject out by a tip streaming mechanism, thus leaving the drop free of particles. On the other hand, when particles aggregate near the equator, it is shown that the drop can be broken into three major droplets, with the middle droplet carrying all particles and the two larger sized droplets on the sides being free of particles. The method also allows us to separate particles for which the sign of the Clausius-Mossotti factor is different, making particles of one type aggregate at the poles and of the second type aggregate at the equator. The former are removed from the drop by increasing the electric field strength, leaving only the latter inside the drop.© 2008 ASME

Self-Assembly of Particles Into 2D Lattices With Adaptable Spacing

It was recently shown in [1–3] that spherical particles floating on a fluid-fluid interface can be self-assembled, and the lattice between them can be controlled, using an electric field. The technique works for a broad range of fluids and particles, including electrically neutral (i.e., uncharged) particles and small particles (micro- and nano-sized particles). In this paper we show that the technique also works for rod-like and cubical particles floating on fluid-fluid interfaces. The method consists of sprinkling particles at a liquid interface and applying an electric field normal to the interface, thus resulting in a combination of hydrodynamic (capillary) and electrostatic forces acting on the particles. It is shown that the relative orientation of two rod-like particles can be controlled by applying an electric field normal to the interface. The lattice spacing of the self-assembled monolayer of rods can be increased by increasing the electric field strength. Furthermore, experiments show that there is a tendency for the rods to align so that they are parallel to each other. The alignment however is not complete. Similarly, the spacing between two cubes, as well as the spacing of a monolayer of cubes, can be adjusted by controlling the electric field strength.Copyright