Vinyl chloride polymerization in microdroplet reactor - Les thèses en ...

Chapter I : Bibliographic review
forces have a huge influence. These forces are of great importance at the microscopic level, while at
macroscopic scale, they are usually ignored in the behalf of forces like pressure or gravity.
So far, only interfaces between two fluids were mentioned in this study. Interfaces of three different
materials are represented by the intersection of their three contact lines. For instance, a droplet of
water on a solid substrate has a triple contact line. Liquids spread differently on a horizontal surface
according to their nature. Actually, it depends also on the third constituent, which is the gas or the
fluid surrounding the drop. Two different situations are imaginable: either the liquid forms a thin film
wetting the solid surface, with the horizontal dimension of the film depending on the initial volume of
liquid. In this case the contact angle θ between the two surfaces is inferior to 90°. In the second case
the liquid forms a droplet and the wetting is therefore partial with θ superior to 90° (Figure I- 13).
Hydrophilic contact θ < 90° Hydrophobic contact θ > 90°
a) b)
water
Glass
θ
water
PDMS
θ
Figure I- 13 :Example of water droplet wetting a surface: a) droplet wetting a glass surface forming a
film; b) water droplet partially wetting a PDMS surface.
Moreover, it has been confirmed that liquid–liquid two-phase flow patterns in microchannels are more
seriously affected by the wettability between the wall and the fluids (Eggers, 1997; Squires and
Quake, 2005). The wettability of the walls of the carrier liquid should be chosen so that the carrier
liquid wets the walls, forcing the incoming immiscible liquid to form droplets. By a clever
combination of multiple junctions, encapsulation of two aqueous liquids in an oil droplet was achieved
by Atencia et al. (2005); Okushima et al. (2004); Zheng et al. (2004) as presented in Figure I- 14.
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