Squeezing superfluid from a stone: coupling superfluidity and elasticity in a supersolid

Starting from the assumption that the normal solid to supersolid (NS-SS) phase transition is continuous, we develop a phenomenological Landau theory of the transition in which superfluidity is coupled to the elasticity of the crystalline lattice. We find that the elasticity does not affect the universal properties of the superfluid transition, so that in an unstressed crystal the well-known anomaly in the heat capacity of the superfluid transition should also appear at the NS-SS transition. We also find that the onset of supersolidity leads to anomalies in the elastic moduli and thermal expansion coefficients near the transition and, conversely, that inhomogeneous lattice strains can induce local variations of the superfluid transition temperature, leading to a broadened transition.     

Microflow and crack formation patterns in drying sessile droplets of liposomes suspended in trehalose solutions

Anhydrobiotic preservation potentially provides a means of long-term storage of mammalian cells in carbohydrate glasses under ambient conditions. During desiccation, sessile droplets of glass-forming carbohydrate solutions exhibit complex phenomena, including fluid flow, droplet deformation, and crack formation, all of which may alter the cell preservation efficacy. Cell-sized liposomes were employed as a model system to explore these phenomena in diffusively dried sessile droplets of trehalose solutions. Two factors were identified that strongly influenced the features of the desiccated droplets: the underlying surface and the liposomes themselves. In particular, the surface altered the droplet shape as well as the microflow pattern and, in turn, the moisture conditions encountered by the liposomes during desiccation. A ring deposit formed when the droplets were dried on polystyrene, as would be expected owing to the capillary flow that generally occurs in pinned droplets. In contrast, when dried on the more hydrophilic glass slide, the resulting droplets were thinner, and the liposomes accumulated near their centers, which was an unexpected result likely owing to the glass-forming nature of trehalose solutions. As might be anticipated given the variations in liposome distribution, the choice of surface also influenced crack formation upon continued drying. In addition to providing a preferential path for drying, such cracks are relevant because they could inflict mechanical damage on cells. The liposomes themselves had an even more profound effect on crack formation; indeed, whereas cracks were found in all droplets containing liposomes, in their absence few of the droplets cracked at all, regardless of the surface type. These complex drying dynamics merit further investigation in the development of anhydrobiotic preservation protocols, particularly with regard to the role therein of surface hydrophobicity and the cells themselves.     

Cell detection and counting through cell lysate impedance spectroscopy in microfluidic devices

Cell-based microfluidic devices have attracted interest for a wide range of applications. While optical cell counting and flow cytometry-type devices have been reported extensively, sensitive and efficient non-optical methods to detect and quantify cells attached over large surface areas within microdevices are generally lacking. We describe an electrical method for counting cells based on the measurement of changes in conductivity of the surrounding medium due to ions released from surface-immobilized cells within a microfluidic channel. Immobilized cells are lysed using a low conductivity, hypotonic media and the resulting change in impedance is measured using surface patterned electrodes to detect and quantify the number of cells. We found that the bulk solution conductance increases linearly with the number of isolated cells contributing to solution ion concentration. The method of cell lysate impedance spectroscopy is sensitive enough to detect 20 cells microL(-1), and offers a simple and efficient method for detecting and enumerating cells within microfluidic devices for many applications including measurement of CD4 cell counts in HIV patients in resource-limited settings. To our knowledge, this is the most sensitive approach using non-optical setups to enumerate immobilized cells. The microfluidic device, capable of isolating specific cell types from a complex bio-fluidic and quantifying cell number, can serve as a single use cartridge for a hand-held instrument to provide simple, fast and affordable cell counting in point-of-care settings.     

Polar stimulation and constrained cell migration in microfluidic channels

Asymmetrical delivery of stimuli to moving cells for perturbing spatially-heterogeneous intracellular signaling is an experimental challenge not adequately met by existing technologies. Here, we report a robust microfluidic platform allowing localized treatment of the front and/or back of moving cells which crawl through narrow channels that they completely occlude. The enabling technical element for this study is a novel design for precise, passive balancing of flow inside the microfluidic device by contacting two fluid streams before splitting them again. The microchannels constrain cell morphology and induce qualitative and quantitative changes in neutrophil chemotaxis that mimic cells crawling through tissues.     

Patterned co-culture of primary hepatocytes and fibroblasts using polyelectrolyte multilayer templates

This paper describes the formation of patterned cell co-cultures using the layer-by-layer deposition of synthetic ionic polymers and without the aid of adhesive proteins/ligands such as collagen or fibronectin. In this study, we used synthetic polymers, namely poly(diallyldimethylammonium chloride) (PDAC) and sulfonated polystyrene (SPS) as the polycation and polyanion, respectively, to build the multilayer films. We formed SPS patterns on polyelectrolyte multilayer (PEM) surfaces either by microcontact printing PDAC onto SPS surfaces or vice-versa. To create patterned co-cultures on PEMs, we capitalize on the preferential attachment and spreading of primary hepatocytes on SPS as opposed to PDAC surfaces. In contrast, fibroblasts readily attached to both PDAC and SPS surfaces, and as a result, we were able to obtain patterned co-cultures of fibroblast and primary hepatocytes on synthetic PEM surfaces. We characterized the morphology and hepatic-specific functions of the patterned cell co-cultures with microscopy and biochemical assays. Our results suggest an alternative approach to fabricating controlled co-cultures with specified cell-cell and cell-surface interactions; this approach provides flexibility in designing cell-specific surfaces for tissue engineering applications.     

Reactivity of AlMe3 with titanium(IV) Schiff base complexes: X-ray structure of [Ti{(μ-Br)(AlMe2)}{(μ-Br)(AlMe2X)}(salen)] · C7H8 (X=Me or Br) and reactivity studies of mono-alkylated [Ti(Me)X(L)] complexes

[TiCl₂(salen)] (1) reacts with AlMe₃ (1:2) to give the heterometallic Ti(III) and Ti(IV) complexes [Ti{(μ-Cl)(AlMe₂)}{(μ-Cl)(AlMe₂X)}(salen)] (X=Me or Cl) (2) and [TiMe{(μ-Cl)(AlCl₂Me)}(salen)] (3). Addition of diethyl ether to 3 affords [Ti(Me)Cl(salen)] (4). The analogous reaction of [TiBr₂(salen)] (5) gives the crystallographically characterised [Ti{(μ-Br)(AlMe₂)}{(μ-Br)(AlMe₂X)}(salen)] (X=Me or Br) (6) and [Ti(Me)Br(salen)] (7) in a single step, whilst the comparable reaction of [TiCl₂{(3-MeO)₂salen}] (8) with AlMe₃ yields [Ti(Me)Cl{(3-MeO)₂salen}] (9) with no evidence of titanium(III) species. Reactivity of both halide and methyl groups of 4 has been probed using magnesium reduction, SbCl₅ and AgBF₄ halide abstraction and SO₂ insertion reactions. Hydrolysis of [Ti(Me)X(L)] complexes affords μ-oxo species [TiX(L)]₂(μ-O) [X=Cl, L=salen (13); X=Br, L=salen (14); X=Cl, L=(3-MeO)₂salen (15)].     

A chirp-compensated, injection-seeded alexandrite laser

A Q-switched alexandrite laser suitable for high-resolution spectroscopy is described which produces pulses of 35 mJ energy in a 10 MHz bandwidth, with a high-purity TEM₀₀ mode. The laser can be synchronised to external event to within 100 ns. The output pulse length is 120 ns and the repetition rate 25 Hz. The laser is injection-seeded by a Ti:sapphire laser operating around 730 nm. To preserve single-mode seeding under external triggering conditions, and to obtain small time-jitter, active cavity length control was used. In order to achieve the 10 MHz output bandwidth, intra-cavity electro-optic modulators were used to compensate for the frequency chirp. The refractive index of alexandrite was found to vary linearly with inversion density with a constant of proportionality equal to 1.6᎒^-24 cm³. In pulsed operation at 730 nm, the index change due to this effect is only partially compensated by changes due to ground state phonon relaxation.     

Electrochemical investigations of oligomers and polymers containing ruthenium- and iron-arene complexes

Cyclic voltammetry was employed to investigate the electrochemical behavior of numerous cyclopentadienyliron (CpFe⁺) and pentamethyl-cyclopentadienylruthenium (Cp*Ru⁺) coordinated oligomers and polymers. The electrochemical behavior of the iron systems indicated the cyclopentadienyliron complexes had isolated redox centers and that changes in the reversibility of the redox couple occurred with changes in solvent and temperature. In contrast, the monometallic ruthenium systems showed large peak separations that suggested slow kinetics on the CV timescale. The cyclic voltammograms of the larger ruthenium-containing oligomers and polymers showed multiple redox steps indicating complex electrochemical behavior.     

Birth, death, and flight: a theory of Malthusian flocks

I study "Malthusian flocks": moving aggregates of self-propelled entities (e.g., organisms, cytoskeletal actin, microtubules in mitotic spindles) that reproduce and die. Long-ranged order (i.e., the existence of a nonzero average velocity (v(r,t))≠0) is possible in these systems, even in spatial dimension d=2. Their spatiotemporal scaling structure can be determined exactly in d=2; furthermore, they lack both the longitudinal sound waves and the giant number fluctuations found in immortal flocks. Number fluctuations are very persistent, and propagate along the direction of flock motion, but at a different speed.