Khristo Khristov

Electric field mediated breakdown of thin liquid films separating microscopic emulsion droplets

The authors present a microfluidic technique for electrically induced breakup of thin films formed between microscopic emulsion droplets. The method involves creating a stationary film at the intersection of two microchannels etched onto a glass substrate. After stabilizing the film, a ramped potential is applied across it. The electrical stresses developed at the film interfaces lead to its rupture above a threshold potential. The potential difference at which the film ruptures assesses the film stability. This approach is employed to demonstrate how surfactant (lecithin) adsorption imparts stability to an ultrathin oil film formed between two water droplets.

'Static' and steady-state foams from ABA triblock copolymers: influence of the type of foam films

Abstract The role of foam films in stability of foams formed from solutions of F108 and P85 (polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO)) ABA block copolymers is analyzed. The Π ( h ) isotherms of disjoining pressure are measured for single foam films, under conditions close to those in the foam to clarify the influence of the type of foam films, common thin; black and bilayer sterically stabilized films on foam stability. The experiments with single foam films are performed with the thin liquid film-pressure balance technique (TLF-PBT) developed on the basis of the microinterferometric method of Sheludko–Exerowa. Foam pressure drop technique (FPDT) is applied to determine foam lifetime at constant pressure. This technique involves applying an increased and regulated pressure in the foam liquid phase (plateau borders) which makes it possible to evaluate the effect of foam film type on foam stability under strictly defined capillary pressure values. A combined pneumatic–mechanical method is used to study foam properties under dynamic conditions. Effect of the polymer type and its concentration, velocities of the mixer rotations and gas flow on the foam volume formed and foam stability are studied. The foam lifetime versus applied pressure ( τ p /Δ P ) dependences along with Π ( h ) isotherms indicate that the surface forces (electrostatic, van der Waals and steric), respectively, the type of foam films, play a decisive role in stability of foams from ABA block copolymers. Under identical conditions the copolymer F108 forms more stable foams than those obtained from the P85 solutions. A parallelism between the properties of dynamic foams and single microscopic foam films is observed. When thicker single foam films are formed (from F108 solutions) the steady-state foam column is higher.

The isoelectric state at the solution/air interface : effect on the stability of foams and foam films from nonionic surfactants

Abstract Model studies of foams and single foam films under close and defined conditions are employed to study the effect of the isoelectric state (pH∗) at the solution/air inteface on the stability (lifetime) of foams from tetraethyleneglycol monodecyl ether C10(EO)4 and eicosaoxyethylene nonylphenol ether, NP20. The parameter pH∗ and the potential of the diffuse electric layer Φo are found by the method of the equilibrium foam film. A metastable zone in which films are both equilibrium and rupturing, is established to appear pH values higher than pH∗. Isotherms of disjoining pressure II(h) are measured for the first time at different pH and its influence on the interaction forces in the foam film is revealed. At constant solution ionic strength the pH change affects strongly the pressure at which films rupture. However, at equal ionic strength and surfactant concentration, the stability of foams dramatically decreases when pH∗ is reached.

Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure

Some salts have been recently shown to decrease the sum frequency generation (SFG) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here, we report a similar trend for the chloride salts of monovalent cations, that is, LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin-film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration dependency by considering the direct ion-water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. Although the former was dominant only at the low-concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broad-band intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye-Hückel lengths using the TFPB technique.

Dotchi Exerowa. Foams Stabilized by Hydrophobically-modified Inulin Polymeric Surfactant

The foams and foam films stabilized by hydrophobically-modified inulin polymeric surfactant (INUTEC SP1) were investigated as a function of electrolyte concentrations. The Foam Pressure Drop Technique was applied to obtain the foam lifetime at constant capillary pressure. The Film Pressure Balance Technique was simultaneously applied to obtain the disjoining pressure as a function on film thickness. The results showed that at INUTEC SP1 concentration of 2× 10−5 mol.dm−3 and constant capillary pressure of 5 kPa the foam lifetime τp was independent of electrolyte concentration in the range from 10−4 to 10−1 mol.dm−3 NaCl. Above 10−1 mol.dm−3 NaCl, the foam lifetime increased rapidly with the increase in NaCl concentration and the most stable foams were produced from solutions containing 1 and 2 mol.dm−3 NaCl. The critical pressure Pcr, foam above which foam collapse (avalanche-like) occurred increased with the increasing of electrolyte concentration and this explained the higher stability of the foam at high NaCl concentrations. The disjoining pressure (Π)-thickness (h) isotherms showed a different behaviour at low and high electrolyte concentrations. At 10−3 mol.dm−3 NaCl, common thin films were obtained and the thickness decreased with increase in the pressure and rupture occurred at about 3 kPa. At 1 mol.dm−3 NaCl, black films were obtained with much smaller thickness that decreased slowly with the increase in Π and rupture occurred at about 2.7 kPa. These results indicated the transition from electrostatic to steric interaction as the electrolyte concentration was increased.