Eric P. Lewandowski

Oriented Assembly of Metamaterials

The creation of complex materials may be aided by advanced colloidal assembly methods involving anisotropically shaped particles. Regular assemblies of colloidal particles have many potential uses from self-assembled electronics to biosensors. Recent advances in particle self-assembly suggest that such assemblies may also provide a simple route to metamaterials at infrared and visible length scales. Such metamaterials may, for example, be used to create cloaking devices or light-based circuits based on manipulations of local optical electric fields rather than on the flow of electrons (1).

Curvature-driven capillary migration and assembly of rod-like particles

Capillarity can be used to direct anisotropic colloidal particles to precise locations and to orient them by using interface curvature as an applied field. We show this in experiments in which the shape of the interface is molded by pinning to vertical pillars of different cross-sections. These interfaces present well-defined curvature fields that orient and steer particles along complex trajectories. Trajectories and orientations are predicted by a theoretical model in which capillary forces and torques are related to Gaussian curvature gradients and angular deviations from principal directions of curvature. Interface curvature diverges near sharp boundaries, similar to an electric field near a pointed conductor. We exploit this feature to induce migration and assembly at preferred locations, and to create complex structures. We also report a repulsive interaction, in which microparticles move away from planar bounding walls along curvature gradient contours. These phenomena should be widely useful in the directed assembly of micro- and nanoparticles with potential application in the fabrication of materials with tunable mechanical or electronic properties, in emulsion production, and in encapsulation.

Capillary interactions between anisotropic particles

Micro and nanoparticle adsorption to and assembly by capillarity at fluid–fluid interfaces are intriguing aspects of soft matter science with broad potential in the directed assembly of anisotropic media. The importance of the field stems from the ubiquitous presence of multiphase systems, the malleability of fluid interfaces, and the ability to tune the interactions of the particles adsorbed on them. While homogeneous spherical particles at interfaces have been well studied, the behavior of anisotropic particles – whether the anisotropy originates from shape or chemical heterogeneity – has been considered only very recently. We review recent advances in the field of anisotropic particles at fluid interfaces, by focusing on particles in the micron and submicron range. We discuss capillary adsorption, orientation, migration, and self-assembly, on planar and curved interfaces, and the rheology of particle-laden interfaces. Prospects for future work and outstanding challenges are also discussed.

Oriented assembly of anisotropic particles by capillary interactions

Partially wet anisotropic particles at an otherwise planar fluid interface create distortions as the interface bends to satisfy boundary conditions at the three phase contact line. Overlapping distortions create capillary interactions that depend strongly on the particle shape. Since the excess surface area associated with the distortions can be locally elevated at certain regions near the particle, the resulting capillary interactions drive assembly in preferred orientations. In this work, arguments relating particle aspect ratio to the preferred orientation for assembly are developed for particles of constant cross section. End-to-end registry of particle faces is achieved by exploiting short-range capillary interactions between particles with complex end faces. Finally, insoluble surfactant monolayers are used to arrest the assembly process.

Rotation and Alignment of Anisotropic Particles on Nonplanar Interfaces

We study the alignment of micron-scale particles at air-water interfaces with unequal principle radii of curvature by optical microscopy. The fluid interface bends to satisfy the wetting conditions at the three phase contact line where the interface intersects the particle, creating deflections that increase the area of the interface. These deflections decay far from the particle. The far field interface shape has differing principle radii of curvature over length scales large compared to the particle. The deflections create excess area which depends on the angle of the particle with respect to the principle axes of the interface. We show that when particles create surface deflections with quadrupolar modes, the particles rotate to preferred orientations to minimize the free energy. In experiment, we focus on uniform surface energy particles, for which quadrupolar modes are forced by the particle shape. Analytical expressions for the torque and stable states are derived in agreement with experiment and confirmed computationally.

Combined Molecular Dynamics Simulation-Molecular Thermodynamic Theory Framework for Predicting Surface Tensions

A molecular modeling approach is presented with a focus on quantitative predictions of the surface tension of aqueous surfactant solutions. The approach combines classical Molecular Dynamics (MD) simulations with a molecular-thermodynamic theory (MTT) [ Y. J. Nikas, S. Puvvada, D. Blankschtein, Langmuir 1992 , 8 , 2680 ]. The MD component is used to calculate thermodynamic and molecular parameters that are needed in the MTT model to determine the surface tension isotherm. The MD/MTT approach provides the important link between the surfactant bulk concentration, the experimental control parameter, and the surfactant surface concentration, the MD control parameter. We demonstrate the capability of the MD/MTT modeling approach on nonionic alkyl polyethylene glycol surfactants at the air-water interface and observe reasonable agreement of the predicted surface tensions and the experimental surface tension data over a wide range of surfactant concentrations below the critical micelle concentration. Our modeling approach can be extended to ionic surfactants and their mixtures with both ionic and nonionic surfactants at liquid-liquid interfaces.

Time, temperature, and mechanical fatigue dependence on underfill adhesion

The observations of the present study show that underfill properties can vary greatly after the initial cure and that these changes can have a significant effect on adhesion. Temperature studies illustrate that underfill adhesion is a strong function of temperature near its glass transition temperature and reveal the importance of adhesion tests at the temperature extremes of the operating conditions and/or reliability thermal cycling tests. Subcritical crack growth results demonstrate that subcritical strain energy release rates do not necessarily scale with critical strain energy release rates. These results are used to create a new adhesion screening methodology that more closely mimics the time, temperature, and mechanical fatigue conditions that an underfilled 1st level package will typically experience.