Interfacial Measurements of Colloidal and Bio-colloidal Systems in Real-Time

  • Paul Coffey

Student thesis: Phd


As advances in thin films are made there is a parallel requirement to develop equipment capable of measuring their properties accurately and consistently. In addition there is a need to understand the parameters that are measured. Typical DPI measurements allow both the refractive index (related to density) and the thickness of the adsorbed layer to be calculated with relatively few assumptions, to a very high precision in real time. This thesis presents the research undertaken to develop multiple path length dual polarisation interferometry (MPL-DPI) and absorption enhanced dual polarisation interferometry (AE-DPI). In addition research is presented that can be used to improve the interpretation of the measured parameters for inhomogeneous films and uniaxial films. The new Interferometric technique MPL-DPI allows the thickness and refractive index of in situ and ex-situ coated ultra-thin films to be measured. The procedures and the mathematics required to calculate the properties of films have been described and the technique verified. The technique was demonstrated using films of PMMA, where good agreement was found with complementary techniques. Furthermore, some key features of MPL-DPI were demonstrated using the measurements of interfacially grafted acrylic acid.The absorption enhanced DPI uses the attenuation of the light within the waveguide, due to the light absorbing properties of a film on its surface. As the composition of a film changed, it was shown that the refractive index and extinction coefficients could be used to separate the mass of the components of the film that absorbed light, from the components of the film that did not. With the use of a semi-uniaxial model, the extra data from the attenuation in two polarisations was used to fit the extraordinary and ordinary extinction coefficients. The extraordinary and ordinary extinction coefficients were used to demonstrate that molecular orientation could be implied. The influence that an inhomogeneous film has on the measured thickness, refractive index and extinction coefficient fitted by homogeneous models were investigated. Formulas are presented to explain the thickness, refractive index and extinction coefficient of the measured film. A formula for the total mass per unit area that uses the refractive index was created to account for films that contain molecules of different refractive index increments (dn/dc's). To separate the mass of the individual molecular species from the total mass per unit area, formulas that use the extinction coefficient were derived so molecules that absorb light could be separated from those that do not. The mass calculated from the refractive index and the mass calculated from the extinction coefficient were also examined for uniaxial films. For uniaxial films both measures of the adsorbed mass were found to be relatively accurate and benefited from a partial cancellation of errors.The accuracy of the measurements made by dual polarisation interferometry technology is systematically examined throughout this thesis. Improvements in the calibration routines are suggested and a procedure for the identification and reduction of errors in the phase and contrast is demonstrated.
Date of Award31 Dec 2011
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorJian Lu (Supervisor)


  • dual polarisation interferometry
  • multiple path length
  • multiple pathlength
  • dual polarization interferometry
  • real time measurements
  • inter-facial measurements
  • inhomogeneous
  • structure
  • absorption enhanced
  • interfacial measurements
  • absorption
  • interferometry
  • adsorption
  • DPI
  • AE-DPI
  • ultrathin films
  • ultra-thin films
  • bio-sensor
  • mass per unit area
  • thin films

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