An experimental technique is introduced for measuring the contact angle of small colloidal particles at planar fluid interfaces. The presented light scattering-based method relies on performing two spectral transmittance measurements: one on a particle monolayer standing at the fluid interface and the other on a dispersion of the same particles in a homogeneous medium. The observed shift between the two transmitted spectra is explained in terms of the phase shift parameter, which is then used to determine the particle position relative to the interface and hence the contact angle. The applicability of the technique is demonstrated through simulations and experiments.
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