Effects of hydrodynamic retardation and interparticle interactions on the self-assembly in a drying droplet containing suspended solid particles.

Self-assembly of particles, suspended in a drying droplet, were studied by the Monte Carlo method. The Brownian diffusion of particles was simulated accounting for the effect of hydrodynamic retardation and interparticle interactions. The model allowed for explaining formation of the "coffee ring" patterns even without accounting for the radial flows towards the three-phase contact line. Morphologies of the drying patterns and their dependence on interparticle interactions and concentration of particles are discussed.

Optical transmission of nematic liquid crystal 5CB doped by single-walled and multi-walled carbon nanotubes.

Comparative studies of optical transmission of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), dispersed in nematic liquid crystal matrix 5CB, were carried out. The data evidence violations of Beer-Lambert-Bouguer (BLB) law both in cell thickness and concentration dependencies. The most striking is the fact that optical transmission dependencies for SWCNTs and MWCNTs were quite different in the nematic phase, but they were practically indistinguishable in the isotropic phase. Monte Carlo simulations of the impact of aggregation on direct transmission and violation of BLB law were also done. The results were discussed accounting for the tortuous shape of CNTs, their physical properties and aggregation, as well as strong impact of perturbations of the nematic 5CB structure inside coils and in the vicinity of CNT aggregates.

Selective extraction from microalgae Nannochloropsis sp. using different methods of cell disruption.

This work studies the extraction of intracellular components from microalgae Nannochloropsis sp. with application of different cell disruption techniques, including pulsed electric field (PEF) (20kV/cm, 1-4ms, 13.3-53.1kJ/kg), high voltage electrical discharge (HVED) (40kV/cm, 1-4ms, 13.3-53.1kJ/kg), ultrasonication (USN) (200W, 1-8min, 12-96kJ/kg), and high pressure homogenization (HPH) (150MPa, 1-10 passes, 150-1500kJ/kg). The data evidence that electrically based disruption techniques (PEF and HVED) allowed selective extraction of water soluble ionic components and microelements, small molecular weight organic compounds and water soluble proteins. Microscopic and sedimentation stability analyses have shown that microalgae cells in HVED-treated suspension were noticeably agglomerated and could be easily settled in centrifuge. The electrically based disruption techniques were ineffective for delivery of pigments (e.g., chlorophylls or carotenoids) and their extraction required subsequent application of more potent disruption techniques. The obtained data have shown that HPH disruption technique was the most effective; however, this mode required the highest power consumption.

Sedimentation stability and aging of aqueous dispersions of Laponite in the presence of cetyltrimethylammonium bromide.

This work discusses the sedimentation stability and aging of aqueous suspensions of Laponite in the presence of cetyltrimethylammonium bromide (CTAB). The concentration of Laponite was fixed at a constant level C(l)=2%wt, which corresponds to the threshold between equilibrium gel IG(1) and glass IG(2) states. The concentration of CTAB C(s) was within 0-0.3 %wt. In the presence of CTAB, the Laponite aqueous suspensions were unstable against sedimentation and separated into the upper and bottom layers (U and B layers, respectively). The dynamic light-scattering technique has revealed that addition of CTAB even at a rather small concentration, C(s)=0.0164 %wt (0.03 cation exchange capacity), induced noticeable changes in the aging dynamics of the U layer. It was explained by equilibration of CTAB molecules that were initially nonuniformly distributed between different Laponite particles. Accelerated stability analysis by means of analytical centrifugation with rotor speed ω=500-4000 rpm revealed three sedimentation regimes: continuous (I, C(s)<0.14 %wt), zonelike (II, 0.140.2%wt). It was demonstrated that the B layer was "soft" in the zonelike regime. The increase of ω resulted in its supplementary compressing and collapse of "soft" sediment above certain critical centrifugal acceleration. The physical nature of the observed behavior, accounting for enhancement of hydrophobic interactions between Laponite particles, is discussed.

Impact of a pulsed electric field on damage of plant tissues: effects of cell size and tissue electrical conductivity.

Efficiency of pulsed electric field (PEF) induced permeabilization at 293 K in selected fruit and vegetable plant tissues (apple, potato, carrot, courgette, orange, and banana) at electric field strength (E) of 400 V·cm(-1), 1000 V·cm(-1) and pulse duration (t(p)) of 1000 µs was studied experimentally. The mean cell radius (〈r〉) was within 30 to 60 µm, and the ratio of electrical conductivities of the intact and damaged tissues (σ(i)/σ(d)) was within 0.07 to 0.79 for the studied tissues. Electroporation theory predicts higher damage for tissue with larger cells; however, the direct correlation between PEF damage efficiency and size of cell was not always observed. To explain this anomaly, a theoretical Monte Carlo model was developed and checked for parameters typical for potato tissue. The model showed a strong dependence of PEF damage efficiency and power consumption (W) on σ(i)/σ(d) ratio. The optimum value of electric field strength (E(opt)) was an increasing function of σ(i)/σ(d), and plant tissues with high σ(i)/σ(d) ratio (σ(i)/σ(d) ≈ 1) required application of a rather strong field (for example, E(opt) ≈ 3000 V·cm(-1) for σ(i)/σ(d) ≈ 0.8). However, the PEF treatment at a lower field (E ≈ 400 V·cm(-1)) allowed regulation of the selectivity of damage of cells in dependence of their size. A good qualitative correspondence between experimental data and simulation results were observed.

Influence of temperature and surfactant on Escherichia coli inactivation in aqueous suspensions treated by moderate pulsed electric fields.

This research employed a conductometric technique to estimate the inactivation kinetics of Escherichia coli cells in aqueous suspensions (1 wt.%) during simultaneous pulsed electric fields (PEF) and thermal treatments. The electric field strength was E=5 kV/cm, the effective PEF treatment time t(PEF) was within 0-0.2 s, the pulse duration t(i) was 10(-3) s, the medium temperature was 30-50 degrees C, and the time of thermal treatment t(T) was within 0-7000 s. The damage of E. coli was accompanied by cell size decrease and release of intracellular components. The synergy between PEF and thermal treatments on E. coli inactivation was clearly present. The non-ionic surfactant Triton X-100 additionally improved its inactivation. The characteristic damage time followed the Arrhenius law within the temperature range 30-50 degrees C with activation energies W=94+/-2 kJ mol(-1) and W=103+/-5 kJ mol(-1) with and without the presence of surfactant, respectively. Relations between cell aggregation, cell zeta-potentials and presence of surfactant were analysed.

Behavior of yeast cells in aqueous suspension affected by pulsed electric field.

This work discusses pulsed electric fields (PEF) induced effects in treatment of aqueous suspensions of concentrated yeast cells (S. cerevisiae). The PEF treatment was done using pulses of near-rectangular shape, electric field strength was within E=2-5 kV/cm and the total time of treatment was t(PEF)=10(-4)-0.1 s. The concentration of aqueous yeast suspensions was in the interval of C(Y)=0-22 (wt%), where 1% concentration corresponds to the cellular density of 2x10(8) cells/mL. Triton X-100 was used for studying non-ionic surfactant additive effects. The electric current peak value I was measured during each pulse application, and from these data the electrical conductivity sigma was estimated. The PEF-induced damage results in increase of sigma with t(PEF) increasing and attains its saturation level sigma approximately sigma(max) at long time of PEF treatment. The value of sigma(max) reflects the efficiency of damage. The reduced efficiency of damage at suspension volume concentration higher than phi(Y) approximately 32 vol% is explained by the percolation phenomenon in the randomly packed suspension of near-spherical cells. The higher cytoplasmic ions leakage was observed in presence of surfactant. Experiments were carried out in the static and continuous flow treatment chambers in order to reveal the effects of mixing in PEF-treatment efficiency. A noticeable aggregation of the yeast cells was observed in the static flow chamber during the PEF treatment, while aggregation was not so pronounced in the continuous flow chamber. The nature of the enhanced aggregation under the PEF treatment was revealed by the zeta-potential measurements: these data demonstrate different zeta-potential signs for alive and dead cells. The effect of the electric field strength on the PEF-induced extraction of the intracellular components of S. cerevisiae is discussed.

Computer simulation of electrical conductivity of colloidal dispersions during aggregation.

The computation approach to the simulation of electrical conductivity of colloidal dispersions during aggregation is considered. We use the two-dimensional diffusion-limited aggregation model with multiple-seed growth. The particles execute a random walk, but lose their mobility after contact with the growing clusters or seeds. The two parameters that control the aggregation are the initial concentration of free particles in the system p and the concentration of seeds psi. The case of psi=1, when all the particles are the immobile seeds, corresponds with the usual random percolation problem. The other limiting case of psi=0, when all the particles walk randomly, corresponds to the dynamical percolation problem. The calculation of electrical conductivity and cluster analysis were done with the help of the algorithms of Frank-Lobb and Hoshen-Kopelman. It is shown that the percolation concentration phi c decreases from 0.5927 at psi=1 to 0 at psi --> 0. Scaling analysis was applied to study exponents of correlation length v and of conductivity t. For all psi>0 this model shows universal behavior of classical 2d random percolation with v approximately t approximately 4/3. The electrical conductivity sigma of the system increases during aggregation reaching up to a maximum at the final stage. The concentration dependence of conductivity sigma(phi) obeys the general effective medium equation with apparent exponent ta(psi) that exceeds t. The kinetics of electrical conductivity changes during the aggregation is discussed. In the range of concentration Pc(phi)

The early stages of Saccharomyces cerevisiae yeast suspensions damage in moderate pulsed electric fields.

The objectives of this study were to investigate the effects of pulsed electric fields (PEF) application to colloidal suspension of Saccharomyces cerevisiae. The electrical conductivity measurements during the PEF-treatment of S. cerevisiae suspensions were used to monitor the extent of cell damages in the intervals of electric field strength E = 3-15 kV/cm and time of PEF treatment t(PEF) = 10(-4) to 1s. At relatively small fields (E < 7.5 kV/cm) the early stages of yeast cells damages were observed. At such treatment conditions, the damage was incomplete and developed at long time of PEF treatment, below the value of E = 7.5 kV/cm which is commonly referred in literature as a threshold for this culture. Data obtained for the disintegration in conductivity experiments were found in good correlation with direct counting of yeast lethality using light microscopy. The PEF-induced lethality of the yeast cells and size flocs increased with the mixing of suspensions and the increase of temperature.

Characteristics of interfacial water affected by proteins adsorbed on activated carbon.

The influence of proteins (bovine serum albumin, BSA, and mouse gamma-globulin, IgG) physically adsorbed or covalently attached via coupling with N-cyclohexyl-N'-(2-morpholinoethyl) carbodiimide methyl-p-toluenesulfonate, CMC, to the surface of activated carbon SCN (spherical carbon with nitrogen) on the mobility of interfacial water molecules was studied by means of 1H NMR spectroscopy with freezing-out of bulk water at 180 < T < 273 K. Relaxation processes in the interfacial non-freezing water were investigated measuring transverse time t2 of proton relaxation dependence on the presence of proteins and CMC. The distribution function of activation free energy of relaxation (with a maximum at 20-22 kJ/mol) was calculated for the protein-water-carbon systems using a regularization procedure and the relationships between t2 and the amounts of the interfacial water unfrozen at T < 250 K assuming the Arrhenius-type dependence for t2(-1) on temperature. The state of unfrozen water in pores of SCN shows that the low temperature relaxation processes occur in narrow pores with half-width X < 1.5 nm.

On the origin of the deviation from the first-order kinetics in inactivation of microbial cells by pulsed electric fields.

A computer model was developed for the estimation of the kinetics of microbial inactivation by pulsed electric field (PEF). The model is based on the electroporation theory of individual membrane damage, where spherical cell geometry and distribution of cell sizes are assumed. The variation of microbial cell sizes was assumed to follow a statistical probability distribution of the Gaussian type. Surviving kinetics was approximated by Weibull equation. The dependencies of two Weibull parameters (shape n and time tau, respectively) versus electric field intensity E and width of cell diameters distribution were studied.

Percolation in models of thin film depositions.

We have studied the percolation behavior of deposits for different (2+1)-dimensional models of surface layer formation. The mixed model of deposition was used, where particles were deposited selectively according to the random (RD) and ballistic (BD) deposition rules. In the mixed one-component models with deposition of only conducting particles, the mean height of the percolation layer (measured in monolayers) grows continuously from 0.898 32 for the pure RD model to 2.605 for the pure BD model, but the percolation transition belongs to the same universality class, as in the two-dimensional (2D) random percolation problem. In two-component models with deposition of conducting and isolating particles, the percolation layer height approaches infinity as concentration of the isolating particles becomes higher than some critical value. The crossover transition from 2D to 3D percolation was observed with increase of the percolation layer height.