  Previous experiments have shown that spherical colloidal particles relax to
equilibrium slowly after they adsorb to a liquid-liquid interface, despite the
large interfacial energy gradient driving the adsorption. The slow relaxation
has been explained in terms of transient pinning and depinning of the contact
line on the surface of the particles. However, the nature of the pinning sites
has not been investigated in detail. We use digital holographic microscopy to
track a variety of colloidal spheres---inorganic and organic, charge-stabilized
and sterically stabilized, aqueous and non-aqueous---as they breach liquid
interfaces. We find that nearly all of these particles relax logarithmically in
time over timescales much larger than those expected from viscous dissipation
alone. By comparing our results to theoretical models of the pinning dynamics,
we infer the area per defect to be on the order of a few square nanometers for
each of the colloids we examine, whereas the energy per defect can vary from a
few $kT$ for non-aqueous and inorganic spheres to tens of $kT$ for aqueous
polymer particles. The results suggest that the likely pinning sites are
topographical features inherent to colloidal particles---surface roughness in
the case of silica particles and grafted polymer "hairs" in the case of polymer
particles. We conclude that the slow relaxation must be taken into account in
experiments and applications, such as Pickering emulsions, that involve
colloids attaching to interfaces. The effect is particularly important for
aqueous polymer particles, which pin the contact line strongly.
