Formation of free films of aqueous solutions of poly(ethylene oxide): The influence of surfactant

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Fall 2010 entry - Anna Wang

References

E. A. van Nierop, D. J. Keupp, and H. A. Stone, "Formation of free films of aqueous solutions of poly(ethylene oxide): The influence of surfactant," Epl 88 (6) (2009)

Introduction

Foams often have proteins, surfactants or a combination of both added to enhance their stability and uniformity. Nierop et al study the free films formed from solutions containing only polyethylene oxide (PEO), and solutions containing PEO and sodium dodecyl sulfate (SDS). The former, to their knowledge, had not been reported in literature until now.

Experimental setup

Figure 1. (a) Schematic of setup. A film forms on the wire frame as the bath containing the solution is lowered. (b) The beam is deflected (centre) initially as it hits the meniscus of the film. The initial film thickness is recorded once the beam is well above the meniscus.

The experimental setup is shown in Figure 1, consisting of a wire frame (either 25μm or 200μm nylon) and a solution bath. Solutions were prepared with varying concentrations of polymer (of one of two molecular weights, 100 000 [‘100k’] and 1 000 000 [‘1M’]), a blue food dye (which absorbs red light), and SDS in half the cases.

The section containing the wire frame for the film to form on was first submerged in the solution; the bath of solution was then lowered so that a thin film formed on the frame. The transmission of incoherent red light from a LED through the film was used to find the film thickness via the Beer-Lambert law (relating the path length and concentration of the dye to the transmitted intensity).

Many other studies use interferometry to study film thickness. Interferometric measurements, however, rely on the film reaching a black-film limit, which does not occur with wires made of the materials used in this experiment – the film often breaks prematurely. Using light absorption as Nierop et al do has the advantages of:

  • direct measurements of film thickness rather than changes in film thickness
  • not being affected by the plumes and convective motions which form during rapidly-draining experiments

Much of the data is plotted against capillary length <math>Ca</math>=µ0<math>U</math>/γ where µ0 is the viscosity, γ is the surface tension and <math>U</math> is the speed that the bath is withdrawn from the wire frame.

Results and conclusions

Figure 2. (a) Typical transmission data. (b) The Beer-Lambert law gives the thickness of the film.
Figure 3. Thinning rate (normalised by final thickness as a function of withdrawal speed). Inset: as a function of <math>Ca</math>

Typical transmission data is shown in Figure 2. The Beer-Lambert law is used to generate the plot of thickness vs time, which shows an abrupt change in thinning rate and hence thinning mechanism at time t3 (corresponding to when the bath stopped moving).

The normalised thinning rate was plotted against withdrawal speed and capillary number (Figure 3) to investigate this further. As the data did not collapse when plotted against capillary number (which depends on the withdrawal speed), the thinning rate is assumed to be independent of viscosity but dependent on withdrawal speed. This was concluded to be consistent with the film stretching during bath withdrawal. Other effects such as drainage would have shown viscosity-dependent drainage rather than the withdrawal-speed dependence observed.

The role of wire thickness and material

It was found that for a solution of 0.5%wt PEO of 1M molecular weight the different wires did not give appreciable differences in data. The remainder of the experiments were performed with the 25µm wire only. Interestingly, the film thickness of PEO films (without SDS) was often found to exceed the thickness of the wires supporting the films.

The role of the surfactant SDS

Figure 4. Two different regimes were observed for the initial film thickness vs capillary number for solutions of SDS and PEO [left = `thickening' and right = `thinning'].

When initial film thickness was plotted against capillary number, two regimes emerged - `thickening' and `thinning' (see Figure 4). The solutions that showed thinning as capillary number increased had higher concentrations of high molecular weight PEO although 0.5%wt PEO showed both types of behaviour. The thinning behaviour observed contradicts existing models, suggesting that other behaviour (eg surfactant-polymer interaction and stretching) may need to be considered.