Wetting phenomena and interactions in phase-separate colloid-polymer mixtures

In this last chapter I will review and integrate the findings of the previous chapters and give suggestions for further research on this topic.In chapter 2 measurements of depletion interactions by means of colloidal probe atomic force microscopy (CP-AFM) are described. We found that the behaviour of the range of the depletion interaction is roughly in agreement with predictions of Fleer et aI. The strength of the depletion interaction is not in agreement with of predictions of Tuinier et al.2 and Louis3 and Bolhuis.4 It is much smaller. We explain this by the fact that the segment-surface interaction for polydimethylsiloxane (PDMS) and a stearylated silica surface is non zero. Comparison of our experimental values with values obtained by Scheutjens Fleer Self Consisted Field theory for different values of Xs leads to a reduced segment surface interaction parameter of 0.41. This corresponds to a decrease to about one order of magnitude of the maximal depletion interaction (see figure 2.13). This strong weakening is in agreement with experimental findings that significantly more PDMS is needed to reach the colloidal liquid-gas binodal than predicted, even if non ideality of the polymers is taken into account.5· 6In chapter 3 we present observations of wetting phenomena in depletion interaction driven, phase-separated colloidal dispersions. These dispersions consist of stearyl coated silica spheres in cyclohexane with PDMS as non-adsorbing polymer. The dynamic contact angle was determined by moving a thin fibre upward or downward through the colloidal liquid gas interface and drawing tangents to the meniscus at the fibre in the three phase contact line. The dynamic contact angle was measured as a function of the velocity of the fibre. From these dynamic contact angles the static contact angle was determined by interpolation to zero velocity. We observed no noticeable hysteresis. The contact angle of the colloidal liquid-gas interface at a solid substrate (coated glass) was determined for a series of compositions. We were able to locate a Cahn wetting transition7 from partial to complete wetting upon approaching the critical point. By this we have tested successfully the well known Cahn prediction regarding wetting transitions for coloid-polyrner mixtures.In the same way we determined the contact angle of the colloidal liquid-gas interface at a soft solid substrate for a series of compositions. A soft solid substrate was obtained by heating a glass fibre in a melt of PDMS. In this way we manipulated the colloidal particle-substrate interaction. Upon approach to the critical point, a transition occurs from partial to complete drying. The region where the wetting transition occurs was compared with that of density functional calculations of Brader et al. and Monte Carlo simulations of Dijkstra et al.8' 9

Bibliographic Details

Main Author:	Wijting, W.K.
Other Authors:	Cohen Stuart, Martien
Format:	Doctoral thesis biblioteca
Language:	English
Subjects:	colloids, mixtures, polymers, wettability, wetting, bevochtigbaarheid, bevochtigen, colloïden, mengsels, polymeren,
Online Access:	https://research.wur.nl/en/publications/wetting-phenomena-and-interactions-in-phase-separate-colloid-poly
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