  We report an experimental study of the crack pattern formed during the drying
of a colloidal suspension. A horizontal fiber, which provides a one
dimensional, boundary-free substrate, is coated by a film of micronic
thickness. The geometry imposes a remarkable annular crack pattern and allowing
precise measurements of the crack spacing over a short range of film thickness
(between 2 and 10 $\mu$m) which varies linearly with the film height. We
compare our experimental data with a model proposed by Kitsunezaki which
suggests that the variation of the crack spacing with the film thickness
depends on the ratio between a critical stress at cracking and a critical
stress for slipping on the substrate. By measuring the friction force of the
colloidal gels on a hydrophobic surface through a cantilever technique, we can
deduce the critical crack stress for these colloidal gels simply by measuring
the crack spacing of the pattern.
