The imbibition of fluids in porous media has been studied widely. Still, processes of preferential flow under gravity due to instability at the wetting front, crucial in groundwater contamination, have yet to be fully understood. Recent theories using dynamic contact angles could describe unstable flow phenomena but have not been proven experimentally. Therefore, infiltration experiments in small sand-filled chambers were conducted to explore the effect of dynamic contact angles. A high-speed camera recorded pore invasion at the unstable imbibition liquid front. Since water moves in a plain in the 2-D finger experiment, the camera can be used to measure the velocity of water. However, the hydrophobic front and back walls possibly altered the flow dynamics. Therefore, these two additional 3-D experiment runs (Run III and IV) were performed to compare the wetting front advancement characteristics in 2-D and 3-D. Specifically, we investigated in 3-D whether the wetting front is discontinuous and water advances one pore at a time, which are the required conditions for the Hofmann-Baver equation.