Simultaneous spreading and evaporation: Recent developments

The recent progress in theoretical and experimental studies of simultaneous spreading and evaporation of liquid droplets on solid substrates is discussed for pure liquids including nanodroplets, nanosuspensions of inorganic particles (nanofluids) and surfactant solutions. Evaporation of both complete wetting and partial wetting liquids into a nonsaturated vapour atmosphere are considered. However, the main attention is paid to the case of partial wetting when the hysteresis of static contact angle takes place. In the case of complete wetting the spreading/evaporation process proceeds in two stages. A theory was suggested for this case and a good agreement with available experimental data was achieved. In the case of partial wetting the spreading/evaporation of a sessile droplet of pure liquid goes through four subsequent stages: (i) the initial stage, spreading, is relatively short (1–2 min) and therefore evaporation can be neglected during this stage; during the initial stage the contact angle reaches the value of advancing contact angle and the radius of the droplet base reaches its maximum value, (ii) the first stage of evaporation is characterised by the constant value of the radius of the droplet base; the value of the contact angle during the first stage decreases from static advancing to static receding contact angle; (iii) during the second stage of evaporation the contact angle remains constant and equal to its receding value, while the radius of the droplet base decreases; and (iv) at the third stage of evaporation both the contact angle and the radius of the droplet base decrease until the drop completely disappears. It has been shown theoretically and confirmed experimentally that during the first and second stages of evaporation the volume of droplet to power 2/3 decreases linearly with time. The universal dependence of the contact angle during the first stage and of the radius of the droplet base during the second stage on the reduced time has been derived theoretically and confirmed experimentally. The theory developed for pure liquids is applicable also to nanofluids, where a good agreement with the available experimental data has been found. However, in the case of evaporation of surfactant solutions the process deviates from the theoretical predictions for pure liquids at concentration below critical wetting concentration and is in agreement with the theoretical predictions at concentrations above it.     

Photodegradation of isotactic poly(1-butene): Multiscale characterization

The effect of photodegradation in isotactic poly(1-butene) (PB-1) have been investigated using rheology, differential scanning calorimetry and infrared spectroscopy. Two commercially available grades of PB-1 with different average molecular weight were chosen. Specimens prepared by compression moulding were UV irradiated in the interval from 0 to 70 h. UV-induced changes in molecular structure have been followed by evolution of rheological properties, thermal properties and degradation by-products. Thermal analysis showed significant changes in crystallization behaviour influencing morphology and resulting thermal properties. Moreover it has been confirmed that the degradation significantly retards the phase transformation. Rheological measurement has been found as an effective method for determination of early stages of photodegradation of PB-1.     

Retention behaviour of imidazolium ionic liquid cations on 1.7 μm ethylene bridged hybrid silica column using acetonitrile-rich and water-rich mobile phases

The potential of 1.7 μm ethylene bridged hybrid silica phase was investigated for the separation of twelve imidazolium-based ionic liquid cations. U-shaped retention profile was observed for all solutes with an increase in retention at both low and high acetonitrile content. Chromatographic behaviour of imidazolium cations in both hydrophilic interaction chromatography (HILIC) and per aqueous liquid chromatography (PALC) modes was studied by varying key parameters such as buffer concentration and pH, acid additive, organic modifier and column temperature. Experimental data provided some evidences that under PALC conditions cationic solutes are retained predominantly by mixed hydrophobic/ion-exchange interactions. In the HILIC mode, both partitioning and ion-exchange interactions are responsible for the retention of solutes. Compared to PALC, HILIC provided significantly higher efficiencies with less or even no peak tailing, better separation selectivity and greater resistance to overload. In PALC mode gradient elution was required to achieve adequate retentivity of all solutes but selectivity was not sufficient to distinguish between solutes with very similar hydrophobicity. In contrast, under HILIC conditions twelve solutes were almost completely resolved in less than 4 min by using isocratic elution. Summarizing, it could be concluded that ethylene bridged hybrid silica column providing a dual retention mechanism offers the possibility of selecting between the two retention modes with opposite separation selectivity, just by changing the composition of the mobile phase.     

The frequency response of dynamic friction: Model comparisons

Any linearised theory of the initiation of friction-excited vibration via instability of the state of steady sliding requires information about the dynamic friction force in the form of a frequency response function for sliding friction. Recent measurements of this function for an interface consisting of a nylon pin against a glass disc are used to probe the underlying constitutive law. Results are compared to linearised predictions from the simplest rate-state model of friction, and a rate-temperature model. In both cases the observed variation with frequency is not compatible with the model predictions, although there are some significant points of similarity. The most striking result relates to variation of the normal load: any theory embodying the Coulomb relation F∝N would predict behaviour entirely at variance with the measurements, even though the steady friction force obtained during the same measurements does follow the Coulomb law.     

Effects of crystal content on the mechanical behaviour of polyethylene under finite strains: Experiments and constitutive modelling

The mechanical stress-strain behaviour of polyethylene (PE) materials under finite strains is studied both experimentally and theoretically. In order to gain insight into the structure and physical properties of investigated PE materials, a series of thermal (DSC and DMTA) and microstructural (small-angle X-ray scattering and AFM) characterizations have been undertaken. The influence of crystallinity on the various features of the tensile stress-strain response is considered over a large strain range, implying thermoplastic-like to elastomer-like mechanical behaviour. A physically-based hyperelastic-viscoplastic approach was adopted to develop a pertinent model for describing the mechanical behaviour of PE materials under finite strains. The semicrystalline polymer is being treated as a heterogeneous medium, and the model is based on a two-phase representation of the microstructure. The effective contribution of the crystalline and amorphous phases to the overall intermolecular resistance to deformation is treated in a composite framework, and coupled to a molecular network resistance to stretching and chain orientation capturing the overall strain hardening response. In order to extract the individual constitutive response of crystalline and amorphous phases, a proper identification scheme based on a deterministic approach was elaborated using the tensile test data of PE materials under different strain rates. Comparisons between the constitutive model and experiments show fair agreement over a wide range of crystallinities (from 15% to 72%) and strain rates. The constitutive model is found to successfully capture the important features of the observed monotonic stress-strain response: the thermoplastic-like behaviour for high crystallinity includes a stiff initial response, a yield-like event followed by a gradual increase of strain hardening at very large strains; for the elastomer-like behaviour observed in the low crystallinity material, the strain hardening response is largely predominant. Strain recovery upon unloading increases with decreasing crystallinity: this is quantitatively well reproduced for high crystallinity materials, whereas predictions significantly deviate from experiments at low crystallinity. Model refinements are finally proposed in order to improve the ability of the constitutive equations to predict the nonlinear unloading response whatever the crystal content.     

A Consumption Behavior Model with Advertising And Word-of-Mouth Effect

It is widely-held belief that people’s consumption behavior is partly determined by advertising and word-of-mouth effect especially in monopolistic competitive market. Owing to this, we propose a mathematical model to interpret consumer behavior under the advertising and word-of-mouth effects, which is divided into continuous and discrete types for dynamic behavior analysis. Our research indicates the continuous model undergoes fold bifurcation, Hopf bifurcation, and degenerate fold-Hopf bifurcation; the discrete model undergoes flip bifurcation and Neimark-Sacker bifurcation. Moreover, bifurcation diagrams are given by using MATLAB to illustrate the model. Based on the theory of Hopf bifurcation or Flip bifurcation, the system undergoes supercritical Hopf bifurcation or Flip bifurcation under certain conditions. This lead to both advertising impacts periodically on consumer behavior, and a short-term reduction cause no effect the public¡s recognition of the brand, which can guide precision advertising investment.     

LiV2O4: electronic,magnetic structure and heavy-fermion behaviour from first principles

First principles studies based on density functional theory (DFT) calculations within the generalized gradient approximations (GGA) and GGA + U approach using the full-potential, augmented plane wave + local orbitals (APW + lo) method, as implemented in the WIEN2k code, have been used to investigate the structural, electronic and magnetic properties of spinel-structure LiV₂O₄, in particular regarding the heavy fermion (HF) behaviour. The calculations were performed for ferromagnetic, anti-ferromagnetic, and ferrimagnetic configurations using two kinds of magnetic structures (tetragonal and rhombohedral). The GGA results showed that the Fermi energy lies in the V 3d (t_2g) bands with 1.5 electrons per V atom occupying this band, and the V 3d bands are separated by a ～1.9 eV energy gap from the O 2p bands and further split into t_2g and e_g bands with a ～1.0 eV energy gap, which are in good agreement with the photoelectron spectra. The GGA + U method indicates that the ground state of LiV₂O₄ is the tetragonal anti-ferromagnetic configuration with metallic character, and ferromagnetic order character at slightly higher energy, which is consistent with experimental result. The geometric frustration and hybridization between 3d (V) and 2p (O) could induce spin fluctuation and help to explain the instability of specific heat, susceptibility and HF behaviour.     

外来种喜旱莲子草的克隆生长特性

通过对外来种喜旱莲子草（Alternanthera philoxeroides（Mart．）Griseb）生长指标的测定和分析，研究了它的克隆生长规律，并探讨丁它的觅食行为．在一个生长季节内，喜旱莲子草生物量增加了近30倍，一级葡萄茎分株数增加了近50倍，最长的一级葡萄茎长度增长了近40倍，一级葡萄茎总长度增长了近13倍；一、二、三级葡萄茎上的总分株数增加了近130倍，3个级别匍匍苇总长度增长了近28倍．通过各级葡萄茎个数，葡萄茎总长度和葡萄茎总分株数比较发现，二级葡萄茎比一、二级葡萄苇生长的都要快，而一级葡萄茎的间隔物平均长度远远大于二级和三级葡萄茎．克隆植物的觅食行为与它的克隆生长紧密相连，从觅食行为来看，喜旱莲子草克隆构型随生长季节的变化说明了它对生长季节具有很强的适应性．这种快速克隆生长的特性和对生长季节的适应性有利于喜旱莲子成功入侵新的生境。