Dispersed Systems

OUR INTRESTS

Research interest is focused on understanding the mechanism of formation and stability of dispersed systems, especially foams. It is showed that magnitude of the stabilizing forces is determined by an actual, often non-equilibrium, adsorption coverage at liquid interfaces. Our primary goal is to determine effects of the diffusion kinetics, solute surface activity, structure of the surfactant molecule, bubble velocity, motion induced non-equilibrium adsorption coverage on the rising bubbles and type of the thin liquid film formed on foams properties. Special interest is focused on the state of adsorption layer at gas/liquid interface under dynamic conditions and influence of the motion induced non-equilibrium adsorption coverage on stability of the thin liquid films formed (foam and wetting films). Influence of surfactant adsorption on local and terminal velocities of bubbles, bubble bouncing and shape pulsations during collisions with various interfaces, dynamics and time-scale of the three phase contact formation (gas-liquid-solid), effect of surfactant and hydrophilic/hydrophobic properties of a solid surface on mechanism and time scale of the three phase contact have been studied. Equilibrium adsorption properties are measured and parameters characterizing the surfactant surface activity are evaluated by fitting the appropriate adsorption isotherms. Surface potential variations at air/liquid interface are determined, as well The studies have an interdisciplinary character and have been carried out in co-operation with research teams from the Max-Planck-Institute for Colloid and Surface Chemistry in Golm/Potsdam and the Institute of Physical Chemistry Bulgarian Academy of Sciences in Sofia and Dept. of Physical Chemistry of Colloids, Henri Poincare University in Nancy, France.

MAIN RESEARCH TOPICS

METHODS

  • Set-up for recording bubble collisions with various interfaces using high speed camera (SpeedCam)
  • Set-up with stroboscopic illumination, for determination of the local and terminal velocities of the bubble
  • Set-up (computerized) for measurements of single bubbles lifetimes at solution surface.
  • Pneumatic and pneumatic-mechanical methods for determination foamability of solutions.
  • Automatic set-up and procedure for swift characterization foamability and foam stability.
  • Direct method for measurements of liquid contents in wet foams.
  • Surface tension measurements (Du Noy ring and stalagmometric methods.
  • Vibrating plate method (Kelvin probe) for the electric surface potential measurements.
  • Circulation apparatus for determination kinetics of multilayer sorption on solid grains.

MAIN ACHIEVEMENTS

  • Showing that air entrapped at hydrophobic solid surfaces facilitated the three phase contact formation
  • Description of the mechanism of formation and stability of wet foams under dynamic conditions.
  • Discovery of a relationship between changes of effective elasticity forces and foamability in homologous series of n-alkanols and fatty acids.
  • Experimental proof that shape of the bubble colliding with various interface can pulsate with frequency over 1000Hz - rapid bubble shape pulsation means variation of the surface area and proves that there is no equilibrium adsorption coverage in such systems.
  • Determination of the amplitude and time scale of the bubble bouncing during its collisions with various interfaces.
  • Determination of the minimum adsorption coverage's needed to immobilize an interface of the rising bubble.
  • Providing the experimental evidence that motion through a surfactant solution leads to dis-equilibration of surfactant surface coverage over the bubble surface.
  • Demonstration that surfactant present in the gas phase lowers the stability of foams and single thin liquid films.
  • Characterization of water contents in foams formed by flotation frothers