Sticky Spheres Research Articles

Vitrification and gelation in sticky spheres.

Glasses and gels are the two dynamically arrested, disordered states of matter. Despite their importance, their similarities and differences remain elusive, especially at high density, where until now it has been impossible to distinguish them. We identify dynamical and structural signatures which distinguish the gel and glass transitions in a colloidal model system of hard and "sticky" spheres. It has been suggested that "spinodal" gelation is initiated by gas-liquid viscoelastic phase separation to a bicontinuous network and the resulting densification leads to vitrification of the colloid-rich phase, but whether this phase has sufficient density for arrest is unclear [M. A. Miller and D. Frenkel, Phys. Rev. Lett. 90, 135702 (2003) and P. J. Lu et al., Nature 435, 499-504 (2008)]. Moreover alternative mechanisms for arrest involving percolation have been proposed [A. P. R. Eberle et al., Phys. Rev. Lett. 106, 105704 (2011)]. Here we resolve these outstanding questions, beginning by determining the phase diagram. This, along with demonstrating that percolation plays no role in controlling the dynamics of our system, enables us to confirm spinodal decomposition as the mechanism for gelation. We are then able to show that gels can be formed even at much higher densities than previously supposed, at least to a volume fraction of ϕ = 0.59. Far from being networks, these gels apparently resemble glasses but are still clearly distinguished by the "discontinuous" nature of the transition and the resulting rapid solidification, which leads to the formation of inhomogeneous (with small voids) and far-from-equilibrium local structures. This is markedly different from the glass transition, whose continuous nature leads to the formation of homogeneous and locally equilibrated structures. We further reveal that the onset of the attractive glass transition in the form of a supercooled liquid is in fact interrupted by gelation. Our findings provide a general thermodynamic, dynamic, and structural basis upon which we can distinguish gelation from vitrification.

Coupling between criticality and gelation in "sticky" spheres: a structural analysis.

We combine experiments and simulations to study the link between criticality and gelation in sticky spheres. We employ confocal microscopy to image colloid-polymer mixtures and Monte Carlo simulations of the square-well (SW) potential as a reference model. To this end, we map our experimental samples onto the SW model. We find an excellent structural agreement between experiments and simulations, both for locally favored structures at the single particle level and large-scale fluctuations at criticality. We follow in detail the rapid structural change in the critical fluid when approaching the gas-liquid binodal and highlight the role of critical density fluctuations for this structural crossover. Our results link the arrested spinodal decomposition to long-lived energetically favored structures, which grow even away from the binodal due to the critical scaling of the bulk correlation length and static susceptibility.

A Review of <i>Bactrocera minax</i> (Diptera: Tephritidae) in China for the Purpose of Safeguarding

The Chinese citrus fly, Bactrocera minax (Enderlein) (Diptera: Tephritidae), is the most destructive citrus pest in many citrus production areas in China. The pest is oligophagous, feeding exclusively on the fruits of citrus plants. The pest is univoltine, with adults emerging during April to May and overwintering as pupae. Evidence suggests that the region of origin of the species might be in the elevated temperate southern Yunnan-Guizhou Plateau, spreading out through China’s major waterway systems. Currently, B. minax occurs in nine citrus producing provinces in China, but mostly prominently in the five south central provinces or municipalities of Guizhou, Hubei, Hunan, Sichuan, and Chongqing. Fruit infestations in these provinces are generally moderate to severe. The species is not reported in the four southern citrus production provinces of Guangdong, Fujian, Zhejiang, and Hainan (island). Orchard sanitation such as collection and treatment of the fallen and hanging infested fruits, mass trapping by using various food-based traps with insecticides, and foliar and ground insecticide sprays are the primary pest management options. Sterile insect technology was studied in the field in the 1980s to 1990s, with promising outcomes. The two highly attractive kairomone lures used for many Bactrocera species, cuelure or methyl eugenol, are not attractive to this species. Hydrolyzed protein, sugar and vinegar mixture, and waste brewer's yeast are the most common lures being used in China. Published data indicated that the quality and efficacy of these lures are inconsistent and disputable. Visual cue lures such as colored sticky spheres are also being used in the field. Preliminary field studies suggested that female rectum extracts demonstrated high attraction to males as well as females of the species. Phytosanitary treatment studies in China focus on irradiation and cold treatment. The outcomes of irradiation appeared promising. When B. minax larvae were treated with an irradiation dose of 50 Gy, no adults emerged from the surviving pupae and larvae failed to pupate when the dose reached 70 Gy. No fruit quality was impacted by the treatment. Results from several small-scale cold treatment studies were less promising and inconsistent. One study indicated that under 0°C constant treatment, no mortality of 3rd instar larvae occurred until day 12, and only 55% mortality occurred by day 22. The scientific and technological gaps in safeguarding US citrus industry from the invasion of this destructive pest include: 1) lack of effective lures for early detection and emergency responses; 2) insufficient work on phytosanitary treatment techniques; and 3) weak definition of and lack of data for B. minax pest free areas in China.

Interaction Heterogeneity can Favorably Impact Colloidal Crystal Nucleation.

Colloidal particles with short-ranged attractions, e.g., micron-scale spheres functionalized with single-stranded DNA oligomers, are susceptible to becoming trapped in disordered configurations even when a crystalline arrangement is the ground state. Moreover, for reasons that are not well understood, seemingly minor variations in the particle formulation can lead to dramatic changes in the crystallization outcome. We demonstrate, using a combination of equilibrium and nonequilibrium computer simulations, that interaction heterogeneity-variations in the energetic interactions among different particle pairs in the population-may favorably impact crystal nucleation. Specifically, interaction heterogeneity is found to lower the free energy barrier to nucleation via the formation of clusters comprised preferentially of strong-binding particle pairs. Moreover, gelation is inhibited by "spreading out over time" the nucleation process, resulting in a reduced density of stable nuclei, allowing each to grow unhindered and larger. Our results suggest a simple and robust approach for enhancing colloidal crystallization near the "sticky sphere" limit, and support the notion that differing extents of interaction heterogeneity arising from various particle functionalization protocols may contribute to the otherwise unexplained variations in crystallization outcomes reported in the literature.

Comparison of commonly-used microwave radiative transfer models for snow remote sensing

This paper reviews four commonly-used microwave radiative transfer models that take different electromagnetic approaches to simulate snow brightness temperature (TB): the Dense Media Radiative Transfer - Multi-Layer model (DMRT-ML), the Dense Media Radiative Transfer - Quasi-Crystalline Approximation Mie scattering of Sticky spheres (DMRT-QMS), the Helsinki University of Technology n-Layers model (HUT-nlayers) and the Microwave Emission Model of Layered Snowpacks (MEMLS). Using the same extensively measured physical snowpack properties, we compared the simulated TB at 11, 19 and 37GHz from these four models. The analysis focuses on the impact of using different types of measured snow microstructure metrics in the simulations. In addition to density, snow microstructure is defined for each snow layer by grain optical diameter (Do) and stickiness for DMRT-ML and DMRT-QMS, mean grain geometrical maximum extent (Dmax) for HUT n-layers and the exponential correlation length for MEMLS. These metrics were derived from either in-situ measurements of snow specific surface area (SSA) or macrophotos of grain sizes (Dmax), assuming non-sticky spheres for the DMRT models. Simulated TB sensitivity analysis using the same inputs shows relatively consistent TB behavior as a function of Do and density variations for the vertical polarization (maximum deviation of 18K and 27K, respectively), while some divergences appear in simulated variations for the polarization ratio (PR). Comparisons with ground-based radiometric measurements show that the simulations based on snow SSA measurements have to be scaled with a model-specific factor of Do in order to minimize the root mean square error (RMSE) between measured and simulated TB. Results using in-situ grain size measurements (SSA or Dmax, depending on the model) give a mean TB RMSE (19 and 37GHz) of the order of 16–26K, which is similar for all models when the snow microstructure metrics are scaled. However, the MEMLS model converges to better results when driven by the correlation length estimated from in-situ SSA measurements rather than Dmax measurements. On a practical level, this paper shows that the SSA parameter, a snow property that is easy to retrieve in-situ, appears to be the most relevant parameter for characterizing snow microstructure, despite the need for a scaling factor.

Structural insights into the multinuclear speciation of tetravalent cerium in the tri-n-butyl phosphate-n-dodecane solvent extraction system.

X-ray and electrochemical studies of organic phases obtained by the extraction of tetravalent cerium, Ce(iv), from aqueous nitric acid (3 M) with tri-n-butyl phosphate (TBP) in n-dodecane reveal a tetranuclear Ce(iv) structural motif. This finding is consistent with the results of previous liquid-liquid extraction (LLE) studies that implicate the aggregation of (Ce-O-Ce)6+ dimers into multinuclear Ce(iv)·TBP solvates. The organic solution structures elaborated here for the Ce(iv)-HNO3-20% TBP-n-C12H26 system are correlated with multiscale phenomena-from the atomic level of the cerium coordination environment to the supramolecular scale of solute aggregates-in the organic phases, which are of relevance to the PUREX (Plutonium Uranium Reduction EXtraction) process. The combination of XANES, EXAFS, and SAXS results indicate the presence of tetranuclear cerium(iv)-oxo core structures in each of the organic phases investigated. In addition to the use of X-ray spectroscopy and scattering for direct metrical details about the organic phase solute speciation, three-phase-electrode differential pulse voltammetry (DPV) of the third phase reveals a wave attributable to Ce(iv) reduction. The electrode potential is consistent with values for the reduction of Ce(iv) in (Ce-O-Ce)6+ dimers in aqueous electrolytes. The Ce(iv) coordination chemistry of the organic solvates is independent of the bulk phenomenon of phase splitting, namely third phase formation. The local, molecular environment of Ce in the organic phase before splitting is identical to those in the two organic phases (the dense third phase and the light phase) after splitting. SAXS data are consistent with the formation of small spherical reverse micelles with core diameters (approx. 6 Å) that can accommodate a tetranuclear Ce(iv) oxo-cluster solvate of TBP. Sticky sphere modeling of the SAXS data for the organic phases with low cerium concentrations (<0.14 M) is consistent with the presence of randomly- and homogenously-dispersed micelles in combination with short-range percolated, associated micelles. At high cerium concentrations (approx. 1.5 M) in the third phase, the SAXS modeling is consistent with correlated, long-range percolated micellar aggregates. The presence of strong inter-micellar interactions (-3 to -5kBT) in all organic phases of the Ce(iv)-HNO3-TBP-n-C12H26 LLE system suggests that the phenomena of phase splitting and third phase inversion are due to liquid precipitation that is dependent solely on the concentration of the tetranuclear Ce solvate.

Generic phase diagram of binary superlattices

Emergence of a large variety of self-assembled superlattices is a dramatic recent trend in the fields of nanoparticle and colloidal sciences. Motivated by this development, we propose a model that combines simplicity with a remarkably rich phase behavior applicable to a wide range of such self-assembled systems. Those systems include nanoparticle and colloidal assemblies driven by DNA-mediated interactions, electrostatics, and possibly, controlled drying. In our model, a binary system of large and small hard spheres (L and S, respectively) interacts via selective short-range ("sticky") attraction. In its simplest version, this binary sticky sphere model features attraction only between S and L particles. We show that, in the limit when this attraction is sufficiently strong compared with [Formula: see text], the problem becomes purely geometrical: the thermodynamically preferred state should maximize the number of [Formula: see text] contacts. A general procedure for constructing the phase diagram as a function of system composition f and particle size ratio r is outlined. In this way, the global phase behavior can be calculated very efficiently for a given set of plausible candidate phases. Furthermore, the geometric nature of the problem enables us to generate those candidate phases through a well-defined and intuitive construction. We calculate the phase diagrams for both 2D and 3D systems and compare the results with existing experiments. Most of the 3D superlattices observed to date are featured in our phase diagram, whereas several more are predicted for future discovery.

Modeling Both Active and Passive Microwave Remote Sensing of Snow Using Dense Media Radiative Transfer (DMRT) Theory With Multiple Scattering and Backscattering Enhancement

In this paper, we incorporate the cyclical terms in dense media radiative transfer (DMRT) theory to model combined active and passive microwave remote sensing of snow over the same scene. The inclusion of cyclical terms is crucial if the DMRT is used to model both the active and passive contributions with the same model parameters. This is a necessity when setting out on a joint active/passive retrieval. Previously, the DMRT model has been applied to active and passive separately, and in each case with a separate set of model parameters. The traditional DMRT theory only includes the ladder terms of the Feynman diagrams. The cyclical terms are important in multiple volume scattering and volume–surface interactions. This leads to backscattering enhancement which represents itself as a narrow peak centered at backward direction. This effect is of less significance in passive remote sensing since emissivity is relating to the angular integral of bistatic scattering coefficients. The inclusion of cyclical terms in first-order radiative transfer (RT) accounts for the enhancement of the double bounce contribution and makes the results the same as that of distorted Born approximation in volume–surface interactions. In this paper, we develop the methodology of cyclical corrections within the framework of DMRT beyond first order to all orders of multiple scattering. The active DMRT equation is solved using a numerical iterative approach followed by cyclical corrections. Both quasi-crystalline approximation (QCA)–Mie theory with sticky spheres and bicontinuous media scattering model are used to illustrate the results. The cyclical correlation introduces around 1 dB increase in backscatter with a moderate snowpack optical thickness of ${\sim}\text{0.2}$ . The bicontinuous/DMRT model is next applied to compare with data acquired in the Nordic Snow Radar Experiment (NoSREx) campaign in the snow season of 2010–2011. The model results are validated against coincidental active and passive measurements using the same set of physical parameters of snow in all frequency and polarization channels. Results show good agreement in multiple active and passive channels.

Non-additive simple potentials for pre-programmed self-assembly.

A major goal in nanoscience and nanotechnology is the self-assembly of any desired complex structure with a system of particles interacting through simple potentials. To achieve this objective, intense experimental and theoretical efforts are currently concentrated in the development of the so-called "patchy" particles. Here we follow a completely different approach and introduce a very accessible model to produce a large variety of pre-programmed two-dimensional (2D) complex structures. Our model consists of a binary mixture of particles that interact through isotropic interactions that enable them to self-assemble into targeted lattices by the appropriate choice of a small number of geometrical parameters and interaction strengths. We study the system using Monte Carlo computer simulations and, despite its simplicity, we are able to self-assemble potentially useful structures such as chains, stripes, and Kagomé, twisted Kagomé, honeycomb, square, Archimedean and quasicrystalline tilings. Our model is designed in such a way that it may be implemented using discotic particles or, alternatively, using exclusively spherical particles interacting isotropically. Thus, it represents a promising strategy for bottom-up nano-fabrication.

Structural aspects of heteropolyacid microemulsions.

Metrical insights from X-ray scattering studies of dense fluid phases (known as "third" phases) in the Keggin heteropolyacid-tri-n-butyl phosphate (TBP)-n-alkane system are provided. Small-angle X-ray scattering (SAXS) experiments reveal inter-acid correlation peaks corresponding to average centre-of-mass to centre-of-mass separations of 18-23 Å between P···P, Si···Si, and Al···Al of H3PW12O40, H4SiW12O40, and H5AlW12O40, respectively, consistent with the presence of TBP solvates that form by hydrogen bonding between the acids and the phosphoryl group of TBP. The Baxter sticky sphere model analyses of the SAXS data reveal identical structures for all the dense phases with inter-cluster interaction energies of ∼5kBT. We demonstrate that the sticky sphere model is an essential paradigm for interpreting SAXS and predicting mesoscale assembly in heteropolyacid microemulsions. The model parameters for the ternary polyoxometalate-amphiphile-oil systems reveal, in rigorous clarity, how the interactions between heteropolyacid solvates underpin their condensation to produce the observed scattering data. Aside from aiding researchers in predicting the physical origins of SAXS in strongly-interacting micellar systems found in natural and engineered settings, such as chemical separations, our study provides mesostructural information that complements previously observed electrochemical behaviours for third phases formed by solvent extraction involving the contact of aqueous electrolytes of dodecatungsto-phosphoric, -silicic, and -aluminic acids with organic solutions (e.g. n-dodecane and n-octane) of TBP, and by simple dissolution of the acid salts of the polyoxometalate hydrates in the same organic solutions.

Ammonium carbonate is more attractive than apple and hawthorn fruit volatile lures to Rhagoletis pomonella (Diptera: Tephritidae) in Washington State.

The apple maggot fly, Rhagoletis pomonella (Walsh), is an introduced, quarantine pest of apple (Malus domestica Borkhausen) in the Pacific Northwest of the United States. In the eastern United States where the fly is native, fruit volatiles have been reported to be more attractive than ammonia compounds to R. pomonella. However, the opposite may be true in the western United States. Here, we determined whether newly identified western apple and western hawthorn fruit volatiles are more attractive than ammonium carbonate (AC) to R. pomonella in apple, black hawthorn, and ornamental hawthorn trees in western Washington State. In all three host trees, sticky red sphere or yellow panel traps baited with AC generally caught more flies than traps baited with lures containing the four newly developed fruit blends (modified eastern apple, western apple, western ornamental hawthorn, and western black hawthorn) or two older blends (eastern apple and eastern downy hawthorn). Fruit volatiles also displayed more variation among trapping studies conducted at different sites, in different host trees, and across years than AC. The results imply that traps baited with AC represent the best approach to monitoring R. pomonella in Washington State.

An Insight into Third-Phase Formation during the Extraction of Thorium Nitrate: Evidence for Aggregate Formation from Small-Angle Neutron Scattering and Validation by Computational Studies

Small-angle neutron scattering (SANS) studies were carried out to compare the aggregation behavior of 1.1 M solutions of tributyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) dissolved in different deuterated diluents, viz., n-dodecane, chloroform, and benzene, during the extraction of Th(IV) from nitric acid medium. The scattering data was treated using the Baxter sticky spheres model. The third phase formed in the case of DHOA displayed higher aggregation tendency compared to that of TBP. These studies have demonstrated that the nature of the diluents plays an important role in the aggregation behavior of the extracted species (reverse micelles). No third phase was observed in the case of chlorinated and aromatic diluents like chloroform and benzene during the extraction of Th(IV) from nitric acid medium. Theoretical calculations were also performed to gain insights into the binding of thorium nitrate with TBP and DHOA models. These calculations suggest that two to three molecules of both DHOA and TBP strongly coordinate to Th(NO3)4. It is noted that the highly charged Th(IV) cations are screened by nitrates and extractants which enables the interaction of second unit of such complex through noncovalent interactions.

Simulating the miscibility of nanoparticles and polymer melts

While the miscibility and spatial dispersion of nanoparticles (NPs) in a polymer melt critically affects the properties of the resulting nanocomposite, little simulation work exists on understanding this critical issue. We use isothermal–isobaric ensemble simulations and show that larger NPs disperse more easily than small NPs, implying the relative dominance of NP–polymer attractions over depletion-induced inter-NP attractions. Similarly, polymer chain length only plays a secondary role, probably because the entropic, depletion-induced inter-NP attractions only occur over length scales comparable to the correlation length in the melt, namely the segment size, σ. Importantly, no NP self-assembly is observed, and the only transition that occurs for polymer systems with large enough NPs (σNP ≥ 6σ) is of a purely, first-order solid–fluid type. This result follows from the fact that the range of effective attractions between the NPs, δ = σ/σNP, is short enough to preclude a vapor–liquid transition. This finding is given more weight since an equivalent sticky sphere model can reproduce the essence of our simulations. The observed behavior is captured by an effective two-body, polymer-mediated, inter-NP interaction potential, a surprising result in light of conventional wisdom in this field which implies the importance of many body effects.

Unique emulsions based on biotechnically produced hydrophobins

The emphasis of this manuscript is on emulsions with gel-like properties based on biotechnically produced hydrophobins. These emulsions are compared to emulsions based on surfactants. Even though the preparation conditions for both emulsion types were the same, the structure and the properties were very different. Homogeneous, gel-like emulsions could be obtained with a protein concentration between 0.02 and 1 wt% and an oil mass fraction of more than 0.65. The gelified state is formed because the protein-covered droplets behave like sticky spheres even when the globules are ionically charged and the long range interaction is repulsive. Conductivity and microscopy measurements showed that the emulsions were of the oil in water (o/w) type. The size of the emulsion droplets depends on the mixing apparatus. With a vortex shaker oil droplets of up to 100 μm diameter were obtained indicating some protein remained in the bulk aqueous phase. With a high pressure homogenizer the emulsion droplets got much smaller and the protein was completely adsorbed at the droplet interface. Interestingly the emulsions aged with time without changing their structure. The aging was a result of the increase of the storage modulus G′. In the case of surfactants no homogeneous stable emulsions could be obtained under the same conditions.

Third-Phase Formation in the Extraction of Phosphotungstic Acid by TBP in n-Octane

The solvent extraction of 12-phosphotungstic acid, also known as 12-tungstophosphoric acid—H3PW12O40, the so-called Keggin heteropolyacid—by 0.73 M (20%v/v) tri-n-butyl phosphate (TBP) in n-octane under conditions comparable to those used previously for the extraction of conventional inorganic mineral acids is described. A simplified phase diagram for the pentanary system comprised of H3PW12O40, HNO3, H2O, TBP, and n-octane reveals an extremely low initial concentration of H3PW12O40 (1.1 mM) at the LOC (limiting organic concentration) condition, far lower than the most effective third-phase-forming inorganic acid, namely HClO4. The results from small-angle neutron scattering (SANS) indicate that the interparticle attraction energy—U(r) calculated through application of the Baxter sticky sphere model to the SANS data at the LOC condition—does not approach the −2 k B T value associated with phase splitting in previous studies of TBP third-phase formation. The third-phase formation model based on attractive interactions between polar cores of reverse micelles, successfully developed for TBP and other extraction systems does not apply to the extraction of H3PW12O40. Rather, the separation of a third-phase from the TBP organic phase stems from the limited solubility of the heavy and highly polar H3PW12O40–TBP species in the alkane diluent.

On the onset of runaway stellar collisions in dense star clusters - II. Hydrodynamics of three-body interactions

The onset of runaway stellar collisions in young star clusters is more likely to initiate with an encounter between a binary and a third star than between two single stars. Using the initial conditions of such three-star encounters from direct N-body simulations, we model the resulting interaction by means of Smoothed Particle Hydrodynamics (SPH). Our code implements new equations of motion that allow for ecient use of non-equal mass particles and is capable of evolving contact binaries for thousands of orbits. We nd that, in the majority of the cases considered, all three stars merge together. In addition, we compare our SPH calculations against those of the sticky-sphere approximation. If one is not concerned with mass loss, then the sticky sphere approach gives the correct qualitative outcome in approximately 75% of the cases considered. Among those cases in which the sticky-sphere algorithm identies only two particular stars to collide, the hydrodynamic calculations nd the same

Preferential positioning of a nanoparticle bound to a polymer: Exact enumeration of a self-avoiding walk chain model

A lattice chain model is extended to investigate the preferential position of a sticky sphere bound to a polymer chain, motivated by wrapping one nanosize core-histone with DNA to form a nucleosome structure. It was shown that the single bound histone is populated in DNA chain ends from the experiment by T. Sakaue et al. [Phys. Rev. Lett. 87, 078105 (2001)]. Here, the possible mechanisms are examined to elucidate such behavior. For neutral chains or ionic chains in high salt concentrations, spheres bound on the middle of chain may trigger conformational constraints to reduce conformational entropy. For ionic chains, the bound sphere can be driven to chain ends if its effective charge and the charge of chain monomers are of like charge. The two-dimensional chain is further studied to mimic the chromosome strongly adsorbed onto surfaces, of which behavior is similar to the three-dimensional case with minor difference due to surface confinement.

Parasitism of Bactrocera oleae (Diptera; Tephritidae) by Psyttalia concolor (Hymenoptera; Braconidae) in the Balearic Islands (Spain)

Abstract Psyttalia concolor (Hymenoptera; Braconidae) was introduced to Spain in the early 1970s to control the olive fruit fly Bactrocera oleae. Previous studies showed that P. concolor was poorly adapted to temperate zones, such as the Balearics. In this study, we demonstrate a high rate of natural parasitism of B. oleae by P. concolor in Majorca (Balearic Islands). Surveys were carried out from August to September 2007 in two organically‐farmed olive orchards and two insecticide‐managed orchards. A total of 250 olive fruits were examined weekly from each of the orchards. Olives were kept under laboratory conditions until the emergence of B. oleae pupae, which were collected and placed in Petri dishes until the emergence of B. oleae and/or P. concolor adults. In addition, red sticky spheres and yellow sticky panel traps were placed in every orchard for monitoring adults of P. concolor. Red spheres captured more individuals of the parasitoid when compared with the yellow panels. Seasonal average parasitism levels of B. oleae pupae by P. concolor were high (22.4% and 23.4%) in the two organically‐farmed orchards and 0% in the insecticide treated orchards. The high rates of parasitism by P. concolor found in the organically‐farmed olive orchards supports the use of this parasitoid for the biological control of B. oleae in the Balearic Islands.

Cooperative activated dynamics in dense mixtures of hard and sticky spheres

The coupled activated dynamics in dense mixtures of repulsive and sticky hard spheres is studied using stochastic nonlinear Langevin equation theory. The effective free energy surface, barriers, saddle point trajectories, and mean first passage times depend in a rich manner on mixture composition, (high) total volume fraction, and attractive interaction strength. In general, there are three types of saddle point trajectories or relaxation pathways: a pure sticky or pure repulsive particle displacement keeping the other species localized, and a cooperative motion involving repulsive and attractive particle displacements. The barrier for activated hopping usually increases with the ratio of sticky to repulsive particle displacement. However, at intermediate values of the displacement ratio it can attain a broad plateau value, and can even exhibit a local maximum, and hence nonmonotonic behavior, at high sticky particle mixture compositions if the attraction strength is modest. The mean first passage, or hopping, times are computed using multidimensional Kramers theory. In most cases the hopping time trends reflect the behavior of the barrier height, especially as the sticky particle attraction strengths become large. However, there are dramatic exceptions associated with cooperative repulsive and attractive particle trajectories where the barriers are high but a greatly enhanced number of such trajectories exist near the saddle point.

Dynamics of Tracer Particles in Gel-like Media

Our previously proposed theory of kinetic arrest and activated barrier hopping in binary mixtures of hard and sticky spheres is applied to the problem of repulsive particle tracer diffusion. For a dynamically frozen matrix, a tracer kinetic arrest diagram is determined using a simplified version of ideal mode coupling theory. The matrix particles cluster more with increasing degree of attraction, resulting in extra free volume for tracer motion that shifts the onset of localization to higher volume fractions. At the nonergodicity transition, the tracer localization length is roughly its diameter and decreases exponentially with increasing matrix volume fraction. If the matrix is formed via thermoreversible gelation of a fluid, then the lifetime of the kinetically arrested state is a critical question. The naïve mode coupling theory localization boundary for a pure sticky particle fluid is computed and compared to the tracer localization boundary under dynamically frozen matrix conditions. Above a threshold degree of attraction, the matrix can be treated as effectively static for times less than the mean activated barrier hopping time, which grows strongly with increasing matrix particle stickiness.