We revisit a classic study [D. S. Hall, Phys. Rev. Lett. 81, 1539 (1998)10.1103/PhysRevLett.81.1539] of interpenetrating Bose-Einstein condensates in the hyperfine states |F=1,m{f}=-1 identical with |1 and |F=2,m{f}=+1 identical with |2 of 87Rb and observe striking new nonequilibrium component separation dynamics in the form of oscillating ringlike structures. The process of component separation is not significantly damped, a finding that also contrasts sharply with earlier experimental work, allowing a clean first look at a collective excitation of a binary superfluid. We further demonstrate extraordinary quantitative agreement between theoretical and experimental results using a multicomponent mean-field model with key additional features: the inclusion of atomic losses and the careful characterization of trap potentials (at the level of a fraction of a percent).

Electrostatic interactions of colloidal particles are typically screened by mobile ions in the solvent. We measure the forces between isolated pairs of colloidal polymer microspheres as the density of bulk ions vanishes. The ionic strength is controlled by varying the concentration of surfactant (NaAOT) in a nonpolar solvent (hexadecane). While interactions are well-described by the familiar screened-Coulomb form at high surfactant concentrations, they are experimentally indistinguishable from bare Coulomb interactions at low surfactant concentration. Interactions are strongest just above the critical micelle concentration, where particles can obtain high surface potentials without significant screening, kappaa << 1. Exploiting the absence of significant charge renormalization, we are able to construct a simple thermodynamic model capturing the role of reverse micelles in charging the particle surface. These measurements provide novel access to electrostatic forces in the limit where the particle size is much less than the screening length, which is relevant not just to the nonpolar suspensions described here, but also to aqueous suspensions of nanoparticles.

Electrostatic forces between small groups of colloidal particles are measured using blinking optical tweezers. When the electrostatic screening length is longer than the interparticle separation, forces are found to be non-pairwise-additive. Both pair and multiparticle forces are well described by the linearized Poisson-Boltzmann equation with constant potential boundary conditions. These findings may play an important role in understanding the structure and stability of a wide variety of systems, from micron-sized particles in oil to aqueous nanocolloids.

Adhesions are multi-molecular complexes that transmit forces generated by a cell’s acto-myosin networks to external substrates. While the physical properties of some of the individual components of adhesions have been carefully characterized, the mechanics of the coupling between the cytoskeleton and the adhesion site as a whole are just beginning to be revealed. We characterized the mechanics of nascent adhesions mediated by the immunoglobulin-family cell adhesion molecule apCAM, which is known to interact with actin filaments. Using simultaneous visualization of actin flow and quantification of forces transmitted to apCAM-coated beads restrained with an optical trap, we found that adhesions are dynamic structures capable of transmitting a wide range of forces. For forces in the picoNewton scale, the nascent adhesions’ mechanical properties are dominated by an elastic structure which can be reversibly deformed by up to 1 µm. Large reversible deformations rule out an interface between substrate and cytoskeleton that is dominated by a number of stiff molecular springs in parallel, and favor a compliant cross-linked network. Such a compliant structure may increase the lifetime of a nascent adhesion, facilitating signaling and reinforcement.

We demonstrate a technique for simultaneously measuring each component of the force vectors and mobility tensor of a small collection of colloidal particles based on observing a set of particle trajectories. For a few-body system of micron-sized polymer beads in oil separated by several particle radii, we find that the mobility tensor is well-described by a pairwise Stokeslet model. This stands in contrast to the electrostatic interactions, which were found to deviate significantly from a pairwise model. The measurement technique presented here should be simple to extend to systems of heterogeneous, non-spherical particles arranged in arbitrary 3D geometries.Typical approaches to understanding the macroscopic behavior of colloidal materials are based on microscopic models including thermal fluctuations and the interactions of pairs of particles. However, collective behaviors of many particles are fundamentally multi-coordinate phenomena that can only be reduced to pair interactions in special circumstances. To that end, techniques for measuring forces in multi-coordinate systems are needed.Most force measurements are based on Hooke's law; forces of interest are inferred by measuring the deflection of carefully calibrated springs. In the microscopic domain, the surface forces apparatus (SFA) and atomic force microscope (AFM) use mechanical cantilevers [1]. Static optical tweezers (OT) measurements use an optical potential. These techniques allow sensitive measurements of interactions between pairs of surfaces, but SFA and AFM are all but impossible to extend to multiparticle force measurements. Static optical tweezers can be used to simultaneously measure forces between sets of particles [2, 3], but such measurements require careful calibration of many traps and may be confounded by optical interactions between particles induced by the tweezers themselves.Hooke's law is not the only way to measure a force; forces may also be inferred from the trajectories of moving particles. If we wanted to measure the force of gravity on a baseball, we could weigh it on a scale, but we could also toss it in the air and measure its trajectory. Astronomers and particle physicists infer forces from trajectories because appropriate calibrated springs are not available. Such dynamical force measurements have the advantage of not requiring any carefully calibrated springs and avoid perturbing the system being studied. Crocker and Grier introduced a dynamical interaction measurement suitable for colloidal systems called Markovian Dynamics Extrapolation (MDE) [4]. In this technique, data are collected by first positioning particles in some initial configuration using optical tweezers, and then switching the tweezers off and recording the trajectories of the freely interacting particles using video microscopy.In Ref.[5], we introduced an alternative technique for extracting forces from similar trajectory data. Whereas the original MDE analysis requires the entire configuration space to be sampled before the interactions in any particular...

When light travels through strongly scattering media with optical gain, the synergy between diffusive transport and stimulated emission can lead to lasing action. Below the threshold pump power, the emission spectrum is smooth and consistent from shot-to-shot. Above the lasing threshold, the spectrum of emitted light becomes spiky and shows strong fluctuations from shot-to-shot. Recent experiments have reported that emitted intensity resembles a power-law distribution (i.e. Lévy statistics). Recent theories have described the emergence of Lévy statistics as an intrinsic property of lasing in random media. To separate intrinsic intensity fluctuations from the motion of scatterers, we compare the statistics of samples with stationary or freely-diffusing scatterers. Consistent with previous reports, we observe Lévy-like statistics when intensity data are pooled across an ensemble of scatterer configurations. For fixed scatterers, we find exponential intensity distributions whose mean intensities vary widely across wavelengths. Lévy-like statistics re-emerges when data are combined across many lasing modes. Additionally, we find strong correlations of lasing peak intensities across wavelengths. A simple mean-field statistical model captures the observed one-and two-point statistics, where correlations in emission intensity arise from competition among all lasing modes for limited gain.

We describe a method to track particles undergoing large displacements. Starting with a list of particle positions sampled at different time points, we assign particle identities by minimizing the sum across all particles of the trace of the square of the strain tensor. This method of tracking corresponds to minimizing the stored energy in an elastic solid or the dissipated energy in a viscous fluid. Our energy-minimizing approach extends the advantages of particle tracking to situations where particle imaging velocimetry and digital imaging correlation are typically required. This approach is much more reliable than the standard squared-displacement minimizing approach for spatially-correlated displacements that are larger than the typical interparticle spacing. Thus, it is suitable for particles embedded in a material undergoing large deformations. On the other hand, squared-displacement minimization is more effective for particles undergoing uncorrelated random motion. In the ESI, we include a flexible MATLAB particle tracker that implements either approach with a robust optimal assignment algorithm. This implementation returns an estimation of the strain tensor for each particle, in addition to its identification.

Lack of robust manufacturing capabilities have limited our ability to make tailored materials with useful optical and thermal properties. For example, traditional methods such as spontaneous self-assembly of spheres cannot generate the complex structures required to produce a full bandgap photonic crystals. The goal of this work was to develop and demonstrate novel methods of directed self-assembly of nanomaterials using optical and electric fields. To achieve this aim, our work employed laser tweezers, a technology that enables non-invasive optical manipulation of particles, from glass microspheres to gold nanoparticles. Laser tweezers were used to create ordered materials with either complex crystal structures or using aspherical building blocks. This project brought together a multidisciplinary team from Sandia, Yale University, and the University of Delaware. This partnership provided a unique educational opportunity for engineering graduate students and postdoctoral researchers, while enabling new nanoengineering manufacturing technologies.