Fluorosurfactants are the most effective compounds to lower the surface tension of aqueous solutions, but their wetting properties as related to low energy hydrocarbon solids are inferior to ...hydrocarbon trisiloxane surfactants, although the latter demonstrate higher surface tension in aqueous solutions. To explain this inconsistency available data on the adsorption of fluorosurfactants on liquid/vapour, solid/liquid and solid/vapour interfaces are discussed in comparison to those of hydrocarbon surfactants. The low free energy of adsorption of fluorosurfactants on hydrocarbon solid/water interface should be of a substantial importance for their wetting properties.
Display omitted
•Trisiloxane aqueous solutions have minimum surface tension 20–25mN/m.•Fluorosurfactant aqueous solutions have minimum surface tension 15–20mN/m.•The first ones spread on moderately hydrophobic hydrocarbons, but the second ones do not.•Probable reason—different energy of adsorption on the solid/liquid interface•The role of adsorption on the solid air/interface is to be cleared.
Nanoparticles are particles with a characteristic dimension below 100 nm. The properties of nanoparticles differ substantially from those of “big” colloidal particles (size bigger than 1 μm) because ...radius of surface forces, which is around 100 nm, is greater than or comparable with the nanoparticles size. The latter means that each nanoparticle could be completely covered by the surface forces of the neighbouring particles at small enough separation. It also means that the well-known Derjaguin approximation cannot be applied directly and some modifications are required. Pairwise interaction between nanoparticles can be used only at an extremely low volume fraction of nanoparticles (below some critical volume fraction, which is ~0.02%), and above this concentration a new theory based on many-particle interactions should be applied, which is yet to be developed. Some recent progress in the area of interaction between nanoparticles is reviewed and the properties of nanosuspensions based on interaction between nanoparticles are described. The authors have not attempted to cover all available literature in the area but instead have tried to underline the fundamental problems in the area which need to be addressed.
Display omitted
•Interaction between nanoparticles•Multi-particles interactions•Experimental techniques•Experimental studies of nanoparticle interactions
The forces acting in colloidal suspensions and affecting their stability and aggregation kinetics are considered. The approximations used for these forces in numerical simulations and the importance ...of the balanced account for both colloidal forces and hydrodynamic interactions are discussed. As an example the results of direct numerical simulations of kinetics of aggregation either with account for hydrodynamic interaction between particles or without it are compared by varying the parameters of the interaction potential between particles and fraction of solid. Simulations are based on the Langevin equations with pairwise interaction between particles and take into account Brownian, hydrodynamic and colloidal forces. It is confirmed that the neglecting of hydrodynamic interaction results in an accelerated growth of aggregates. The results of numerical simulations of aggregation kinetics are compared with well known analytical solutions.
Aggregates formed at the fraction of solid 0.32; time of aggregation 20s; depths of the potential well Umin=6kT. Display omitted
► Forces affecting stability and aggregation kinetics of colloidal suspensions. > Brownian dynamic simulations of aggregation kinetics. ► Reversible aggregation with constant average size of aggregates at 4-8 kT. ► Aggregates growth rate is overestimated by neglecting of hydrodynamic interactions.
Evaporation of sessile droplets Kovalchuk, N.M.; Trybala, A.; Starov, V.M.
Current opinion in colloid & interface science,
08/2014, Letnik:
19, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Recent developments in the studies of evaporation of liquid droplets placed on a solid substrate are reviewed for the droplet size typically larger than 1μm, so that kinetics effects of evaporation ...are neglected. The attention is paid to the limits of applicability of classical diffusion model of evaporation, effect of substrate, evaporation of complex fluids and applicability for its description of the theory developed for pure liquids, and hydrothermal waves accompanying evaporation.
Display omitted
•Recent developments in the studies of evaporation of liquid droplets placed on a solid substrate are presented.•Complex liquids and hydrothermal waves accompanying evaporation.•Limits of applicability of classical diffusion model of evaporation.
Surfactants are broadly used to improve wetting properties of aqueous formulations. The improvement is achieved by essential reduction of liquid/air and solid/liquid interfacial tensions resulting in ...the decrease of contact angle. For moderately hydrophobic substrates, there is a range of surfactants providing complete wetting of substrate. With the decrease of substrate surface energy, this range of surfactants reduces very quickly and only trisiloxane surfactant solutions are capable to wet completely such highly hydrophobic substrates as polypropylene and parafilm. That is why these surfactants are referred to as superspreaders. The most intriguing feature of wetting surfactant solutions is their ability to spread much faster than pure liquids with spread area, S, being proportional to time, t, S~t, as compared to S~t0.2 for pure liquids, which wet completely the solid substrate. Trisiloxane surfactant solutions spread faster than other aqueous surfactant solutions, which also provide complete wetting, being superspreaders in the sense of spreading rate as well. The mechanism of fast spreading of surfactant solutions on hydrophobic substrates and much higher spreading rates for trisiloxane solutions are to be explained. Below the available experimental data on superspreading and surfactant-enhanced spreading are analysed/summarised, and possible mechanisms governing the fast spreading are discussed.
Display omitted
•Comparison of spreading performance of various surfactants•Possible mechanisms of fast spreading of surfactant solutions on hydrophobic substrates•Available experimental data on superspreading and surfactant-enhanced spreading•Synergism in spreading of mixed solutions of cationic and anionic surfactants
Bovine leukemia virus (BLV) is an oncogenic retrovirus of the
genus, which causes persistent infection in its natural hosts - cattle, zebu, and water buffalo with diverse clinical manifestations ...through the defeat of B-cells. The BLV proviral genome, along with structural genes (
, and
), includes nonstructural ones (
(for miRNAs). We have shown in our previous data the association of some
(for BLV miRNA) alleles with leukocyte (WBC - white blood cell) number in BLV-infected cows. Multifunctional properties of Tax protein have led us to an assumption that
gene/Tax protein could have too population variations related to WBC counts. Here we report about several
alleles/Tax protein variants, which have a highly significant association with an increase or a decrease of WBC number in BLV-infected cows. We have provided evidence that Tax A, H variants (
alleles) are correlated with reduced WBC counts at the level of BLV-negative groups of animals and thus could be the feature of the aleukemic (AL) form of BLV infection. We suggest this finding could be used in BLV testing for the presence of Tax A, H in the proviral DNA consider such strains of BLV as AL ones, and because of this, minimize the clinical losses due to BLV infection in cattle.
Display omitted
Mixed solutions of cationic and anionic surfactants show considerable synergism in their wetting behaviour, but their spreading is affected considerably by the phase separation ...processes. The valuable information about wetting properties of synergetic mixtures can be obtained by using mixtures in which phase separation occurs at concentrations above cmc.
Spreading properties of mixed solutions of cationic and anionic surfactants over highly hydrophobic substrate such as polyethylene are investigated and compared with those for trisiloxane superspreader. Experiments are performed at relative humidity of 40% and 80%. Interfacial tension at water/air and water/alkane interfaces is measured to explain spreading performance.
Catanionic solutions can wet hydrophobic substrates nearly as effective as solutions of trisiloxane superspreader. The spreading factor reaches 70% of that of superspreader for the most effective mixed solution. The spreading slows down earlier at high surfactant concentrations. At room humidity (40%) spread area has a maximum vs concentration. However, the maximum was not observed at higher humidity 80%. Humidity does not affect the short-time spreading rate, but it influences considerably the time when spreading slows down. The spreading rate of mixed solutions is smaller than that of superspreader despite the same spreading exponent α=0.5.
Display omitted
► The surface activity of SDS is increased due to presence of bivalent ions. ► In presence of bivalent ions double chain surfactants like Ca(DS)2 can be formed. ► The hydrolysis of ...SDS is enhanced in NaCl solutions, producing dodecanol. ► Bivalent ions further enhance the SDS hydrolysis in solution. ► SDS in hard water represents a multicomponent surfactant solution.
The complementary drop and bubble profile analysis and maximum bubble pressure tensiometry are used to measure the dynamic surface tension of aqueous SDS solutions in the presence of hardness salts (CaCl2 and MgCl2 in the ratio of 2:1 at concentrations of 6 and 40FH). The presence of hardness salts results in an essential increase of the SDS adsorption activity, which indicates the formation of Ca(DS)2 and Mg(DS)2 in the SDS solutions. The surface tension isotherms of SDS in presence of Ca(DS)2 and Mg(DS)2 are described using the generalised Frumkin model.
The presence of hardness salts accelerates the ageing of SDS solutions as compared with the addition of 0.01M NaCl due to a faster hydrolysis and hence formation of dodecanol. These results are used to estimate the possible concentration of dodecanol in the studied SDS solutions.
The buoyant bubble profile method with harmonic surface oscillations is used to measure the dilational rheology of SDS solutions in presence of hardness salts in the frequency range between 0.005Hz and 0.2Hz. The visco-elasticity modulus in the presence of hardness salts is higher as compared with its values in the presence of 0.01M NaCl additions. The ageing of SDS solutions leads to an essential increase of the visco-elastic modulus.
Display omitted
► Experimental results indicate a significant delay in onset of Marangoni convection. ► The simulation results show an immediate Marangoni convection. ► Quasi-static isotherm ...equations cannot be simply applied for the mobile interfaces.
The inflow pattern of liquid into a droplet is studied experimentally using a surface active dye and compared with results of CFD simulations. The results show visual agreement between experiments and simulations. The CFD simulations show also good agreement with the surface tension measured by drop profile analysis tensiometry (PAT). The inflow of the surfactant induces a Marangoni instability caused by the local arrival of the surfactant at the drop surface. The onset of this Marangoni instability observed experimentally has a delay of about 10s when compared with the simulation results. Different scenarios are discussed, including a boundary layer barrier, a kinetic-controlled adsorption mechanism, the way of renewing a mobile interface, and a critical Marangoni number required for the onset of such instability. It turns out that besides the commonly defined critical Marangoni number, the main reason for delay of Marangoni instability is possibly the mobility of the drop surface which can prevent the establishment of a quasi static adsorbed layer required for the appearance of such surface effect.
PDF
