  We have studied the kinetic roughening of an oil--air interface in a forced
imbibition experiment in a horizontal Hele--Shaw cell with quenched disorder.
Different disorder configurations, characterized by their persistence length in
the direction of growth, have been explored by varying the average interface
velocity v and the gap spacing b. Through the analysis of the rms width as a
function of time, we have measured a growth exponent beta ~= 0.5 that is almost
independent of the experimental parameters. The analysis of the roughness
exponent alpha through the power spectrum have shown different behaviors at
short (alpha_1) and long (alpha_2) length scales, separated by a crossover
wavenumber q_c. The values of the measured roughness exponents depend on
experimental parameters, but at large velocities we obtain alpha_1 ~= 1.3
independently of the disorder configuration. The dependence of the crossover
wavenumber with the experimental parameters has also been investigated,
measuring q_c ~ v^{0.47} for the shortest persistence length, in agreement with
theoretical predictions.
