Wetting in Color: Colorimetric Differentiation of Organic Liquids with High Selectivity
[under construction: Ian Burgess, Spring 2012]
Every liquid has a surface tension. Likewise, every liquid will display a contact angle on an ideal flat surface with a given surface chemistry (where this contact angle could be 0). Simple diagnostics that differentiate liquids based on surface tension and wettability have the potential therefore to be useful in a broad range of scenarios. This paper presents a form of colorimetry based on wetting, where organic liquids induce the appearance of color patterns in an indicator material that are highly sensitive to the liquids' wetting properties.
Central to the operation of this device is the porous inverse-opal films (see B. Hatton et al, Proc. Nat. Acad. Sci. USA 107, 10354-10359 (2010)) that serve as its structural basis. The highly regular porosity of this structure serves two purposes: i) it is responsible for the film's iridescent color (when the pores are air-filled) that disappears when the pores are filled with liquid (due to refractive-index matching); and ii) it facilitates the highly selective wetting that this device exploits.
The paper starts by discussing the correlation between the symmetry of the structure and the selectivity of wetting expected. The pore structure consists of a face-centered cubic lattice of spherical air pores in a silica monolith. Each pore is connected by roughly circular openings, called "necks" in the paper. Due to the discontinuous and re-entrant curvature at the necks, liquid fronts show a propensity to pin at these necks, preventing the wetting of the structure. Sufficiently wetting liquids (sufficiently low intrinsic contact angle) will be able to overcome pinning and penetrate the structure. As shown in Fig. 1, the smaller necks, the higher propensity for pinning. The size of a neck defines a critical intrinsic contact angle for fluid penetration. When a structure