Shahab Shojaei-Zadeh

Capillary interactions between spherical Janus particles at liquid–fluid interfaces

We study the interactions of spherical Janus particles at non-equilibrium orientations adsorbed at flat oil–water interfaces. At equilibrium, the orientation of Janus spheres results in each hemisphere being exposed to its more favored fluid. Experimental observations suggest that some of these particles may take a tilted orientation at the interface, giving rise to energetic inter-particle interactions. The interface shape around the particles is obtained numerically by minimizing the total surface energy of the system comprising the interface and particle-fluid regions. We quantify these interactions through evaluation of capillary energy variation as a function of the separation distance between the particles. Depending on the relative orientations of neighboring particles, attractive or repulsive forces appear between Janus spheres. The interactions are found to be dipolar in nature, with a pair potential inversely proportional to the third power of the inter-particle distance. We find that Janus spheres with similar orientations undergo a relative realignment in the interface plane in order to minimize the capillary energy. We also evaluate the dependence of capillary forces on the size and degree of amphiphilicity between two sides of the particle. This study can provide new insight into understanding the interactions and assembly of Janus particles of random orientations at liquid–fluid interfaces.

Highly crosslinked poly(dimethylsiloxane) microbeads with uniformly dispersed quantum dot nanocrystals

This study demonstrates how luminescent semiconductor nanocrystals (quantum dots or QDs) can be dispersed uniformly in a poly(dimethylsiloxane) (PDMS) matrix by polymerizing a mixture of the prepolymer oligomers and the nanocrystals with a relatively large concentration of crosslinking molecules. A microfluidic device is used to fabricate PDMS microbeads embedded with the QDs by using flow focusing to first form monodisperse droplets of the prepolymer/crosslinker/nanocrystal mixture in a continuous aqueous phase. The droplets are subsequently collected, and heated to polymerize them into solid microbead composites. The degree of aggregation of the nanocrystals in the matrix is studied by measuring the nonradiative resonance energy transfer (RET) between the nanocrystals. For this purpose, two quantum dots are used with maxima in their luminescence emission spectrum at 560 nm and 620 nm. When the nanocrystals are within the Förster radius (approximately 10 nm) of each other, exciton energy cascades from the QDs which emit at the shorter wavelength to the QDs which emit at the longer wavelength. This energy transfer is quantified, for two concentration ratios of the prepolmer to the crosslinker, by measuring the deviation of the microbead luminescence spectrum from a reference spectrum obtained by dispersing the QD mixture in a solvent (toluene) in which the nanocrystals do not aggregate. For a low concentration of crosslinking molecules relative to the prepolymer (5:1 by weight prepolymer to crosslinker), strong RET is observed as the emission of the 620 nm QDs is increased and the 560 nm QDs is decreased relative to the reference. In the emission spectrum for a higher concentration of crosslinkers (2:1 by weight prepolymer to crosslinker), the resonance energy transfer is less relative to the case of the low concentration of crosslinkers, and the spectrum more closely resembles the reference. This result indicates that the increase in the crosslinker concentration has reduced the nanocrystal aggregation in the cured polymer. The use of crosslinking can serve as a general paradigm for forming, from a prepolymer/nanoparticle mixture, a composite in which the particles are not aggregated. Under the usual conditions the entropic cost to a linearly growing polymer chain of surrounding nanoparticles forces them to aggregate; crosslinking kinetically entraps the particles and circumvents this aggregation driving force. The QD/polymer composite microbeads fabricated in this study find applications in bead-based platforms for high-throughput, multiplexed screening, where the emission spectrum of the QD luminescence can be used as a spectral barcode to label the beads. For microbeads in which the nanocrystals are uniformly dispersed, this barcode is undistorted by energy transfer, and is easily read.

Tilting and Tumbling of Janus Nanoparticles at Sheared Interfaces

We investigate the response of a single Janus nanoparticle adsorbed at an oil-water interface to imposed shear flows using molecular dynamics simulations. We consider particles of different geometry, including spheres, cylinders, and discs, and tune their degree of amphiphilicity by controlling the affinity of their two sides to the fluid phases. We observe that depending on the shape, amphiphilicity, and the applied shear rate, two modes of rotational dynamics takes place: a smooth tilt or a tumbling motion. We demonstrate that irrespective of this dynamic behavior, a steady-state orientation is eventually achieved as a result of the balance between the shear- and capillary-induced torques, which can be tuned by controlling the surface property and flow parameters. Our findings provide insight on using flow fields to tune particle orientation at an interface and to utilize it to direct their assembly into ordered monolayers.

Role of Geometry and Amphiphilicity on Capillary-Induced Interactions between Anisotropic Janus Particles

We study the capillary interactions between ellipsoidal Janus particles adsorbed at flat liquid-fluid interfaces. In contrast to spherical particles, Janus ellipsoids with a large aspect ratio or a small difference in the wettability of the two regions tend to tilt at equilibrium. The interface deforms around ellipsoids with tilted orientations and thus results in energetic interactions between neighboring particles. We quantify these interactions through evaluation of capillary energy variation as a function of the spacing and angle between the particles. The complex meniscus shape results in a pair interaction potential which cannot be expressed in terms of capillary quadrupoles as in homogeneous ellipsoids. Moreover, Janus ellipsoids in contact exhibit a larger capillary force at side-by-side alignment compared to the tip-to-tip configuration, while these two are of comparable magnitude for their homogeneous counterparts. We evaluate the role of particles aspect ratio and the degree of amphiphilicity on the interparticle force and the capillary torque. The energy landscapes enable prediction of micromechanics of particle chains, which has implications in predicting the interfacial rheology of such particles at fluid interfaces.

Thermal characterization of thin film Cu interconnects for the next generation of microelectronic devices

With the dramatic scaling of the transistors, the important issues like RC delay, electromigration failure and heat dissipation emerge, which need to be addressed urgently. Substitution of copper for aluminum has been suggested to reduce the RC delay of interconnects. While the electrical and mechanical properties of thin copper films have been extensively investigated; their thermal characterizations have received less attention. The lateral thermal conductivity of a 144 nm thick copper film is measured using the electrical resistance Joule heating and thermometry in a suspended bridge. The thermal conductivities at 300 K and 450 K are 240 and 280 W/m-K, respectively, which is smaller than the corresponding bulk values. The impact of the interconnect dimension and thermal conductivity on the self-heating is investigated as a function of interconnect via density. It is concluded that for via separation distances less than 5 /spl mu/m, the combination of Cu interconnect and vias can significantly reduce the average temperature rise in multilayer interconnects.

Molecular simulation of translational and rotational diffusion of Janus nanoparticles at liquid interfaces

We perform molecular dynamics simulations to understand the translational and rotational diffusion of Janus nanoparticles at the interface between two immiscible fluids. Considering spherical particles with different affinity to fluid phases, both their dynamics as well as the fluid structure around them are evaluated as a function of particle size, amphiphilicity, fluid density, and interfacial tension. We show that as the particle amphiphilicity increases due to enhanced wetting of each side with its favorite fluid, the rotational thermal motion decreases. Moreover, the in-plane diffusion of nanoparticles at the interface becomes slower for more amphiphilic particles, mainly due to the formation of a denser adsorption layer. The particles induce an ordered structure in the surrounding fluid that becomes more pronounced for highly amphiphilic nanoparticles, leading to increased resistance against nanoparticle motion. A similar phenomenon is observed for homogeneous particles diffusing in bulk upon increasing their wettability. Our findings can provide fundamental insight into the dynamics of drugs and protein molecules with anisotropic surface properties at biological interfaces including cell membranes.

Detailed modeling of temperature rise in giant magnetoresistive sensor during an electrostatic discharge event

With further miniaturization of the giant magnetoresistive (GMR) heads, the electrostatic discharge (ESD) failure has become the primary reliability issue in manufacturing of these sensors. In this article, the thermal response of the GMR read head to excessive current/voltage during an ESD event is investigated numerically, using a three-dimensional (3D) finite element analysis. Unlike the previous studies, the thermal properties of the GMR, Al2O3 gap, and shield layers used in the simulation are the experimentally measured values, which are different from the bulk values. The simulation results show that temperature in the GMR element sharply increases as the GMR dimensions are reduced, indicating the future GMR heads will be more susceptible to the ESD damages. In addition, thermal properties of the GMR elements and the gap materials play key roles in the accurate prediction of the temperature field in a GMR head during ESD events. The simulations are performed for both the current-in-plane (CIP) and p...

Particle adsorption at polydimethylsiloxane (PDMS)/water interfaces in the presence of a cross-linking reaction.

The adsorption of polymeric particles at the interface of spherical drops of polydimethylsiloxane (PDMS) with water is studied in presence of a cross-linking reaction. Hydrophobic colloidal particles are first uniformly dispersed in water with the help of a nonionic surfactant. The PDMS droplets are then introduced to this dispersion. The particles adsorb to the PDMS-water interface, as the dispersion is being stirred, while the PDMS undergoes a cross-linking reaction at an elevated temperature. The effect of parameters such as particle mass fraction in the dispersion, PDMS cross-linker concentration, and curing temperature on the adsorption behavior and surface coverage of the resulting beads are examined. The surface coverage increases with increasing particle mass fraction, but decreases as the curing temperature or cross-linker concentration is increased. Using an Arrhenius equation, we correlate the adsorption efficiency to the reaction rate and show that the surface coverage of the beads increases logarithmically with the reaction time.

An Aqueous-Based Approach for Fabrication of PVDF/MWCNT Porous Composites

In this paper, we demonstrate the fabrication of conductive porous polymers based on foaming of an aqueous dispersion of polymeric particles and multi-walled carbon nanotubes (CNT). By tuning the surface energy of the constituents, we direct their preferential adsorption at the air-liquid (bubble) interface or within the liquid film between the bubbles. Sintering this bi-constituent foam yields solid closed-cell porous structure which can be electrically conductive if CNT are able to form a conductive path. We measure transport (electrical and thermal), mechanical, and morphological properties of such porous structures as a function of CNT loading and the method used for their surface functionalization. For a fixed polymer volume fraction, we demonstrate the limit in which increasing CNT results in decreasing the mechanical strength of the sample due to lack of adequate polymer-CNT bond. Such lightweight conductive porous composites are considered in applications including EMI shielding, electrostatic discharge protection, and electrets.

Behavior of Janus Particles at Liquid Interfaces

We study the capillary-induced interactions and configuration of spherical and non-spherical Janus particles adsorbed at flat liquid-fluid interfaces. For Janus spheres, the equilibrium orientation results in each hemisphere being exposed to its more favored fluid. However, experimental observations suggest that some of these particles may take a tilted orientation at the interface, giving rise to a deformed interface. On the other hand, Janus ellipsoids with a large aspect ratio or a small difference in the wettability of the two regions tend to tilt even at equilibrium. The overlap of deformed menisci results in energetic interactions between neighboring particles. We numerically calculate the interface shape around the particles by minimizing the total surface energy of the system comprising of the interface and particle-fluid regions. We quantify these interactions through evaluation of capillary energy variation as a function of the orientation and separation distance between the particles. We find that Janus spheres with similar orientations undergo a relative realignment in the interface plane in order to minimize the capillary energy. In case of ellipsoidal particles, the particles assemble in a preferred side-by-side configuration. We evaluate the role of anisotropy and degree of amphiphilicity on the inter-particle force and the capillary torque. The results can be used to predict the migration and oriented assembly of Janus particles with various geometrical and surface properties at liquid-fluid interfaces.Copyright