Rhombic Patterns Near a Bicritical Point in Periodically Forced Surface Waves

We present here a study of selection of rhombic patterns close to a bicritical point at the onset of primary surface instability in viscous fluids under two-frequency vertical vibration. Rhombic patterns appear to be natural at the primary instability in the form of a bicritical point if the ratio of driving frequencies is selected properly. We present two different patterns which may be accessible in a Faraday experiment.