Interfacial rheology

Interfacial rheology is a branch of rheology that studies the flow of matter at the interface between a gas and a liquid or at the interface between two immiscible liquids. The measurement is done while having surfactants, nanoparticles or other surface active compounds present at the interface. Unlike in bulk rheology, the deformation of the bulk phase is not of interest in interfacial rheology and its effect is aimed to be minimized. Instead, the flow of the surface active compounds is of interest.

The deformation of the interface can be done either by changing the size or shape of the interface. Therefore interfacial rheological methods can be divided into two categories: dilational and shear rheology methods.

Interfacial dilational rheology
Pulsating drop method for dilatational interfacial rheology

In dilatational interfacial rheology, the size of the interface is changing over time. The change in the surface stress or surface tension of the interface is being measured during this deformation. Based on the response, interfacial viscoelasticity is calculated according to well established theories:[1][2]

where

  • |E| is the complex surface dilatational modulus
  • γ is the surface tension or interfacial tension of the interface
  • A is the interfacial area
  • δ is the phase angle difference between the surface tension and area
  • E’' is the elastic (storage) modulus
  • E’'' is the viscous (loss) modulus

Most commonly, the measurement of dilational interfacial rheology is conducted with an optical tensiometer combined to a pulsating drop module. A pendant droplet with surface active molecules in it is formed and pulsated sinusoidally. The changes in the interfacial area causes changes in the molecular interactions which then changes the surface tension.[3] Typical measurements include performing a frequency sweep for the solution to study the kinetics of the surfactant.[4]

In another measurement method suitable especially for insoluble surfactants, a Langmuir trough is used in an oscillating barrier mode. In this case, two barriers that limit the interfacial area are being oscillated sinusoidally and the change in surface tension measured.[5]

Interfacial shear rheology
Interfacial shear rheology with the needle method

In interfacial shear rheology, the interfacial area remains the same throughout the measurement. Instead, the interfacial area is sheared in order to be able to measure the surface stress present. The equations are similar to dilatational interfacial rheology but shear modulus is often marked with G instead of E like in dilational methods. In a general case, G and E are not equal.[6]

Since interfacial rheological properties are relatively weak, it causes challenges for the measurement equipment. For high sensitivity, it is essential to maximize the contribution of the interface while minimizing the contribution of the bulk phase. The Boussinesq number, Bo, depicts how sensitive a measurement method is for detecting the interfacial viscoelasticity.[6]

The commercialized measurement techniques for interfacial shear rheology include magnetic needle method, rotating ring method and rotating bicone method.[7] The magnetic needle method, developed by Brooks et al[8]., has the highest Boussinesq number of the commercialized methods. In this method, a thin magnetic needle is oscillated at the interface using a magnetic field. By following the movement of the needle with a camera, the viscoelastic properties of the interface can be detected. This method is often used in combination with a Langmuir trough in order to be able to conduct the experiment as a function of the packing density of the molecules or particles.

Applications

When surfactants are present in a liquid, they tend to adsorb in the liquid-air or liquid-liquid interface. 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. Many biological systems like pulmonary lung surfactant and meibum are dependent on interfacial viscoelasticity for their functionality.[9]

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. 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. Also enhanced oil recovery efficiency can be optimized.[10]

See also

References
  1. Miller, Reinhard. Liggieri, L. (Libero) (2009). Interfacial rheology. Brill. ISBN 978-90-04-17586-0. OCLC 907184149.CS1 maint: multiple names: authors list (link)
  2. Miller, Reinhard; Ferri, James K.; Javadi, Aliyar; Krägel, Jürgen; Mucic, Nenad; Wüstneck, Rainer (2010-05-01). "Rheology of interfacial layers". Colloid and Polymer Science. 288 (9): 937–950. doi:10.1007/s00396-010-2227-5. ISSN 0303-402X.
  3. Rane, Jayant P.; Pauchard, Vincent; Couzis, Alexander; Banerjee, Sanjoy (2013-04-16). "Interfacial Rheology of Asphaltenes at Oil–Water Interfaces and Interpretation of the Equation of State". Langmuir. 29 (15): 4750–4759. doi:10.1021/la304873n. ISSN 0743-7463. PMID 23506138.
  4. Scientific, Biolin. "Interfacial Rheology | Measurements". www.biolinscientific.com. Retrieved 2019-12-20.
  5. Bykov, A.G.; Loglio, G.; Miller, R.; Noskov, B.A. (2015). "Dilational surface elasticity of monolayers of charged polystyrene nano- and microparticles at liquid/fluid interfaces". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 485: 42–48. doi:10.1016/j.colsurfa.2015.09.004. ISSN 0927-7757.
  6. Krägel, Jürgen; Derkatch, Svetlana R. (2010). "Interfacial shear rheology". Current Opinion in Colloid & Interface Science. 15 (4): 246–255. doi:10.1016/j.cocis.2010.02.001.
  7. Renggli, D.; Alicke, A.; Ewoldt, R. H.; Vermant, J. (2020). "Operating windows for oscillatory interfacial shear rheology". Journal of Rheology. 64 (1): 141–160. doi:10.1122/1.5130620. ISSN 0148-6055.
  8. Brooks, Carlton F.; Fuller, Gerald G.; Frank, Curtis W.; Robertson, Channing R. (1999). "An Interfacial Stress Rheometer To Study Rheological Transitions in Monolayers at the Air−Water Interface". Langmuir. 15 (7): 2450–2459. doi:10.1021/la980465r. ISSN 0743-7463.
  9. Leiske, Danielle L.; Leiske, Christopher I.; Leiske, Daniel R.; Toney, Michael F.; Senchyna, Michelle; Ketelson, Howard A.; Meadows, David L.; Fuller, Gerald G. (2012). "Temperature-Induced Transitions in the Structure and Interfacial Rheology of Human Meibum". Biophysical Journal. 102 (2): 369–376. Bibcode:2012BpJ...102..369L. doi:10.1016/j.bpj.2011.12.017. PMC 3260664. PMID 22339874.
  10. Ayirala, Subhash C.; Al-Saleh, Salah H.; Al-Yousef, Ali A. (2018). "Microscopic scale interactions of water ions at crude oil/water interface and their impact on oil mobilization in advanced water flooding". Journal of Petroleum Science and Engineering. 163: 640–649. doi:10.1016/j.petrol.2017.09.054. ISSN 0920-4105.
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