Research

Cooperative Interfacial Assembly of Nanoparticle Surfactants for Microfluidic Droplet Generation

Nanoparticles assembly at fluid interfaces can be utilized to form surfactants. Using amine-terminated polymer in oil and carboxylic acid-functionalized nanoparticles dispersed in water, nanoparticle surfactants can be formed in-situ due to the carboxylate-amine electrostatic interactions. The in-situ formed nanoparticle surfactants could enable the generation of structured liquids. The shaped fluids represent a unique state of matter where kinetically trapped packing of NP-surfactants at interfaces leads to the kinetic trapping and shaping of liquids on the macroscopic length scale. If dispersions of carboxy-functionalized NPs in water is injected into a flowing solution of amine-terminated polymer in toluene, e.g. in a microfluidic flow-focusing junction, then water droplets will be formed. We have investigated the droplet shape as a function of the size and concentration of the functionalized NPs and the concentration and molecular weight of the end-functionalized polymer, the viscosities of the fluids, and the flow rate. These studies will, of course, rely on a basic understanding of the kinetics of the formation and assembly of the NP-surfactants at the interface that is investigated by pendant drop tensiometry.

References:

  • A. Toor, T. Feng and T. P. Russell, "Self-Assembly of Nanomaterials at Fluid Interfaces ", The European Physical Journal E, vol. 39 (5), pp. 1-13, 2016.
  • A. Toor, and T. P. Russell, "In-Situ Formed Nanoparticle Surfactants for Microfluidic Droplet Generation ", Lab on a Chip, Under-preparation.

Effect of Nanoparticle Surfactants on the Break-up of Free-falling Water Jets

Structured liquids, whose spatial arrangement can adapt and respond to external stimuli, represent a revolutionary materials platform for next-generation energy technologies, such as batteries, photovoltaics, and thermoelectrics. Liquids can be structured by the jamming of the interfacial assemblies of the nanoparticle (NP) surfactants. Due to the interactions between functional groups on nanoparticles dispersed in one liquid and polymers having complementary end-functionality dissolved in a second immiscible fluid, the anchoring of a well-defined number of polymer chains onto the NPs leads to the formation of NP-surfactants that assemble at the interface and reduce the interfacial energy. Microfluidic techniques provide a simple and versatile route to produce one liquid phase in a second where the shape of the dispersed liquid phase can range from droplets to tubules depending on the flow conditions and the interfacial energies.

In this study, the effect of NP-surfactants on Plateau-Rayleigh instabilities of a free-falling jet of an aqueous dispersion of carboxylic acid functionalized silica NPs into a toluene phase containing amine-terminated polymer is investigated. As the aqueous suspension of nanoparticles is drawn from an orifice into a solution of an amine-terminated polydimethylsiloxane (PDMS), NP-surfactants form at the oil-water interface. The interfacial tension undergoes a significant reduction due to the formation and assembly of NP surfactants at the liquid-liquid interface. To understand the jet breakup dynamics, experiments on water-oil systems with and without NP surfactants were performed, varying fluid flow rate, jet diameter, and NP surfactant concentration. NP surfactants were found to significantly affect the breakup of laminar liquid jets, resulting in longer jet breakup lengths and dripping to jetting flow transitions.

References:

  • A. Toor, B. Helms, and T. P. Russell, “Effect of Nanoparticle Surfactants on the Break-Up of Free-Falling Water Jets During Continuous Processing of Reconfigurable Structured Liquid Droplets", Nano Letters, 2017.

Polymer Nanocomposites for High Performance Dielectrics

Materials with high dielectric constants have drawn increasing interests in recent years for their important applications in capacitors, actuators, and high energy density pulsed power. Particularly, polymer-based dielectrics, owing to their properties like high electric breakdown field, low dielectric loss, flexibility and easy processing are excellent candidates. In order to enhance the dielectric constant of polymer materials, typically, high dielectric constant fillers materials are added to the polymer.

Previously, ferroelectric and conductive fillers have been mainly used. However, such systems suffered from various limitations. For example, composites based on ferroelectric materials like barium titanate exhibited high dielectric loss and poor saturation voltages. Conductive fillers are used in the form of powder aggregates, and have shown 10-100 times enhancement in dielectric constant. But, due to the use of particle aggregates, the dielectric loss was significant. Also, agglomerates limited the volume fraction of fillers in polymer and hence, the ability to achieve superior dielectric constants.

We propose the use of metal nanoparticle fillers to enhance the dielectric properties of the base polymer while minimizing dielectric loss by preventing nanoparticle agglomeration. Novel combinations of materials, which use 5 nm diameter metal nanoparticles embedded inside high breakdown strength polymer materials are evaluated. High breakdown strength polymer materials are chosen to allow further exploration of these materials for energy storage applications. The focus is on obtaining uniform dispersion of nanoparticles with no agglomeration by utilizing appropriate ligands/surface functionalizations on the gold nanoparticle surface. Use of ligand coated metal nanoparticles will enhance the dielectric constant while minimizing dielectric loss, even with the particles closely packed in the polymer matrix.

In summary, a novel nanocomposite material is designed and synthesized using polyvinylpyrrolidone (PVP) functionalized gold nanoparticles embedded inside a polvinylidene fluoride (PVDF) polymer matrix. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy and ultramicrotoming techniques were used for the material characterization of the nanocomposite material. The electron microscopy images of the nanocomposite films demonstrate no particle agglomeration up to 12.5 wt% particle concentration. Nanocomposite films without voids and phase-separation between the particles and polymer phase were successfully synthesized. The nanocomposite films show high dielectric constant and low dielectric loss, i.e., a dielectric constant value of 22 and loss tangent of 0.14 at 1 kHz was measured at a particle concentration of 12.5 wt%.

In addition, a photodefinable nanocomposite with enhanced dielectric properties is successfully developed using 5 nm gold nanoparticles and SU-8 polymer, for embedded capacitor, optical devices, and power electronics applications. These results provide evidence that enhanced dielectric permittivity can be obtained while maintaining the low dielectric loss for polymer composites embedded with coated metal nanoparticles. Such solid-state dielectric materials would enable energy storage solutions having both high energy storage capacity and power transfer rate.

References:

  • A. Toor, H. So, and A. P. Pisano, "Improved Dielectric Properties of Polyvinylidene Fluoride Nanocomposite Embedded with Poly(vinylpyrrolidone)-Coated Gold Nanoparticles" ACS Applied Materials & Interfaces, 9 (7), pp. 6369-6375, 2017.
  • A. Toor, H. So, and A. P. Pisano, “Enhanced Dielectric Permittivity and Low Dielectric Loss Properties of SU-8 Photopolymer Based Nanocomposites”, Applied Surface Science, 2017.
  • A. Toor and A. P. Pisano, "Gold Nanoparticle/Polymer Composites with Improved Particle Dispersion", IEEE International Conference on Nanotechnology, IEEE NANO 2015, pp. 706-709.
  • A. Toor, J. C. Cheng and A. P. Pisano, "Synthesis and Characterization of Gold Nanoparticle/SU-8 Polymer based Nanocomposite", IEEE International Conference on Nano/Micro Engineered and Molecular Systems, IEEE NEMS 2014, pp. 664-668.

Precision Rapid Prototyping of Silicon Masters using Laser Ablation Fabrication

The use of commercial low power laser systems for fabrication of microfluidic components in silicon has opened up many new avenues of research. Presented in this work is a novel wafer clamping mechanism that allows the creation of silicon masters with high accuracy, precision and repeatability. A kinematic coupling based wafer positioning system was designed and integrated in the laser tool. The wafer alignment system improved the accuracy by 74% in X and 81% in Y directions. This technology has been developed for a wide range of aspect ratios, with depths ranging from 30 µm to 400 µm. The influence of process parameters (the focal height, frequency and wavelength) on the etch quality was empirically determined. The laser tool was capable of creating features as small as 33 µm in width with a 1.7 µm standard deviation. Overall, the work explored the range and critical size of the features that can be fabricated in a silicon master using a low power fiber laser. The ability to successfully transfer the patterns from a silicon master to polydimethylsiloxane (PDMS) was also evaluated.

References:

  • A. Toor, L. S. Smith and A. P. Pisano, "Precision Rapid Prototyping of Silicon Masters using Laser Ablation Fabrication ", Journal of Micromechanics and Microengineering, Under-submission.