Session B4: Condensed Matter Physics I

Identification of Ion-Pair Structures in Solution by Vibrational Stark Effects

Ion pairing is fundamental to a wide range of sciences and technologies including batteries and organic photovoltaics. Ions in solution are known to inhabit multiple possible states, including free ions (FI), contact ion pairs (CIP), and solvent-separated ion pairs (SSIP). However, it is difficult to distinguish between these states in solutions of organic radicals and nonmetal electrolytes. In this work we report evidence for the formation of SSIPs in low-polarity solvents and distinct measurements of CIP, SSIP, and FI, by using the nitrile infrared (IR) band of a nitrile-substituted fluorene radical anion. Use of time-resolved IR detection following pulse radiolysis allowed us to unambiguously assign the peak of the FI. In the presence of nonmetal electrolytes, two distinct red-shifted peaks were observed and assigned to the CIP and SSIP. The assignments are interpreted in the framework of the vibrational Stark effect (VSE) and are supported by the solvent dependence of ion-pair populations, the observation of a cryptand-separated sodium ion pair that mimics the formation of SSIPs, and electronic structure calculations. Additionally, we show that a blue-shift of the nitrile IR band due to the VSE can be induced in a nitrile-substituted fluorene radical anion by covalently tethering it to a metal-chelating ligand that forms an intramolecular ion pair upon reduction and complexation with sodium ion. These results combined show that we can identify ion-pair structures by using the VSE, including the existence of SSIPs in a low-polarity solvent.   

Droplet Dynamics of a Flowing Emulsion System in a Narrow Channel

The inner workings of glassy systems have long been a topic of interest for soft material scientists. Similarities between the jamming behavior of emulsions and the glass transition of glassy systems have prompted the conjecture that they might share the same underlying mechanism. Here we study a dense oil-in-water emulsion system forced to flow through a narrow microchannel. By matching the index of refraction of the two phases, the internal dynamics of the droplets could be imaged in a confocal microscope. At low flow speeds, the velocity along the edge of the microchannel was not significantly different than the average droplet velocity in the bulk, suggesting a nearly plug flow. By contrast the droplets near the edge experienced more movement perpendicular to the flow indicating the fluidization effect of the confining walls.   

Polyelectrolyte Complexes in Solution: A Molecular Dynamics Study

Ion-containing polymers are important materials in food, energy, and water industry. To better understand how the structures of polyelectrolytes, we employ molecular dynamics simulations to study the complexation of oppositely charged polyelectrolyte chains in a solution. Polyelectrolytes are modeled as bead spring chains of charged Lennard-Jones (LJ) particles. Solvent is treated as a dielectric background. Explicit counterions and salt ions, treated as single LJ beads, are included in the model. Our simulations show that the structure of the resulting complex formed by polyanions (PAs) and polycations (PCs) depends on the charge ratio ($x)$ between PC and PA chains and the salt concentration (C$_{s})$. At $x $near 1 and small C$_{s}$, all chains condense into a macroscopic drop. Our results show that the macroscopic drop phase exists only in a small range of C$_{s}$ and is destabilized when C$_{s}$ is increased. When $x$ is smaller than 1, the number of PC chains is insufficient to neutralize all PA chains. When $x$ is large than 1, one or several PA chains form complexes with abundant PC chains that are positively charged. Our simulations suggest that the macroscopic drop phase become unstable when $x$ deviates from 1 in both negative and positive directions.   

Bubble Shape and Stress Induced Rearrangements in a Bubble Raft

Soap bubbles floating at an air-water interface experience shape deformations as a result of surface tension and hydrostatic forces. In this experiment we experimentally investigate the shape of individual gas bubbles freely floating at the interface as a function of their gas volume. As bubbles increase in volume, their shape goes from spherical to hemispherical. We empirically determine the dependence of the capillary rise and dome radius as a function of the diameter of the bubbles. Due to shape deformations, collections of interfacial bubbles tend to aggregate and form stable jammed packings. We investigate the stress distributions in these floating aggregates by placing them between parallel plates and subjecting them to uniaxial compression while capturing the deformations and rearrangements with a video camera. We find that under compression, the stress distribution is inhomogeneous and characterized by strings of more stressed bubbles around less stressed regions reminiscent of force chains in granular materials. Finally bubble rearrangements are mapped against the stress map to inform their correlation with local stress variations in the bubble raft.   

Conduit Bound Sound Propagation Separation Model

In a fluid flowing in a conduit if a sound source is placed up-flow and a sound wave is sent propagating through the conduit in the same direction as the fluid flow the sound propagates at the speed-of-sound plus the velocity of the fluid. When a sound source is placed down-flow the sound propagates at the speed-of-sound minus the flow velocity of the fluid. Once the up-flow and down-flow propagation speeds are determined it is a trivial to determine the velocity flow and speed-of-sound of the fluid. It has been shown that one measurement of phase cannot remove the interference of other reflected waves, and leads to an erroneous calculations of flow velocity and speed of sound. This presentation describes a method employing an array of transducers to measure the propagating sound at multiple locations. The phase of the propagating wave form can be determined by employing a fast 2D FFT of time domain signals acquired at several locations in the directions of the fluid flow. This leads to a far more robust and accurate determination of the up-flow and down-flow propagation speeds, which leads to a more accurate calculation of the velocity flow and speed-of-sound of the fluid.   

Atomistic simulations of laser ablation of metals in liquid environment

Laser ablation of metal target in liquid environment is actively used for generation of clean colloidal nanoparticles with unique shapes and functionalities. The fundamental mechanisms responsible for the nanoparticle formation are not fully understood. In this presentation, we report the results of the first atomistic simulations of laser ablation of metal targets in liquid environment. A model combining a coarse-grained representation of liquid, a fully atomistic description of laser interactions with metal targets, and acoustic impedance matching boundary conditions is developed. In contrast to nanoparticle generation in vacuum, the phase decomposition in the liquid environment is partially suppressed and the hot mixture of metal vapor and clusters ejected from the irradiated target is localized in a low-density mixing region where the liquid is brought to the supercritical state. The main nanoparticle formation mechanism is found to be the condensation of clusters from the metal vapor, followed by coalescence and coarsening within the supercritical water region. The results of the simulations support the notion of the important role of the cavitation bubble in the process of nanoparticle formation. The simulations also predict that larger nanoparticles can be generated at sufficiently high laser fluences via Rayleigh-Taylor instability of the plume -- supercritical liquid interface, leading to the formation of a bimodal nanoparticle size distribution commonly observed in experiments.