Just as rheology is the study of flow in bulk fluids, interfacial rheology is the study of the flow properties of liquid interfaces.

These flow properties are important for many products and industries since they determine the behavior and stability of suspensions, emulsions, froth and foams. For example, paints, pharmaceuticals, cosmetics, personal care and food products all require optimum composition and stability for their performance and shelf life. The petrochemical and mineral processing industries often struggle with problems associated with emulsion and froth stability.

Surfactants, polymers, lipids, proteins or particles can influence the flow and stability properties of liquid interfaces in a formulation. Dilatational interfacial rheological parameters can be determined by changing the area of the interface, and measuring the change in interfacial tension by drop shape analysis. The Theta Optical Tensiometer equipped with a piezoelectric transducer (PD-200) is a powerful combination for studying flow behavior and determining interfacial rheological parameters of liquid interfaces.

 

Dilatational interfacial rheology measurements 

When surface-active molecules are present in a liquid, they tend to adsorb in the liquid-vapor and/or liquid-liquid interface as illustrated in Figure 1. Interfacial rheology deals with the response of the adsorbed interfacial layer on the deformation. The response depends on the layer composition, and thus interfacial rheology is relevant in many applications in which adsorbed layer play a crucial role, for example in development surfactants, foams and emulsions.

AT_AdsorptionLayer_Illustration.jpgFigure 1: Adsorbed molecular layers in liquid-vapor and liquid-liquid interfaces

Interfacial rheology enables the study of surfactant kinetics, and the viscoelastic properties of the adsorbed interfacial layer correlate well with emulsion and foam stability. Surfactants and surface active polymers used for stabilising emulsions and foams in food and cosmetic industries. Surfactants are amphiphilic, which means that they consist of hydrophilic and hydrophobic parts. In the adsorption process, the surfactant molecules orientate themselves so that the hydrophilic head is in water and hydrophobic head in oil or vapor (Figure 1). Polymers, such as proteins, are surface active and tend to adsorb at the interface, where they can change conformation and influence the interfacial properties. Natural surfactants like asphaltenes and resins stabilize water-oil emulsions in crude oil applications, and by understanding their behavior the crude oil separation process can be enhanced.

The deformation of the interfacial layer can be caused either by changing the layer size or shape, and this corresponds to elasticity and viscosity of dilation and shear, respectively. In a dilatational interfacial rheology, the deformation is typically caused by dilation/compression of a hanging droplet and interfacial tension follows the change in area of the droplet when surface active molecules are present (Figure 2). 

AT_OscillatingDrop_CompressionDilatation.jpgFigure 2A: In dilatational interfacial rheology, deformation to the adsorbed molecular layer is caused by dilation/compression of the droplet.

AT_GraphInterfacialRheology_PD-200.jpgFigure 2B: Interfacial tension (grey circles) follows the change in droplet area (turquoise squares).

Attension Theta combined with pulsating droplet module, PD-200, enables dilatational interfacial measurements, where interfacial tension and droplet area changes are recorded as a function of time and frequencies. From this data viscoelastic properties of adsorbed molecular layer are calculated according well established theories1 2 resulting in parameters below:

  • |E| — a complex surface dilatational modulus
  • d — phase angle difference between the surface tension and drop area results.
  • E’ — elastic (storage) modulus
  • E’’ — viscous (loss) modulus

PD-200 and OneAttension software (Figure 3) offers a practical approach to dilatational interfacial measurements. The software includes automatic multipoint measurement of viscoelastic properties as a function of frequency and time. A unique volume-from-image function in OneAttension can compensate for droplet evaporation and real-time analysis enables pre-testing of suitable frequency levels and experimental follow-up. PD-200 has  great solvent resistance and can be installed in few minutes.

AT_Theta_OScillatingDrop_OneAttensionScreen.jpgFigure 3: Pulsating droplet volume experiment with the OneAttension Software live analysis.

For more information, see:

Application Note 11 — Pulsating Drop Technique to Characterize Surfactant Behavior in Flotation Process

References

  1. Miller, R. Liggieri, L. (Eds), Progress in colloid and interface science series: Interfacial rheology” (2009) Vol 1., Brill, Leiden.

  2. Miller, R., Ferri, J.K., Javadi, A., Krägel, J., Mucic, N. and Wüstneck, R., “Rheology of interfacial layers” (2010) Colloid. Polym. Sci., 288:937-350.