Methods and apparatus for forming 2-dimensional drop arrays

A microfluidic circuit comprising:

• (a) a drop forming region comprising a channel or nozzle having an inlet for receiving a sample, a constant cross-sectional area, and an outlet that opens into a step region, the channel region or nozzle being rectangular with a height of 1 to 20 μm and a width of 2 to 60 μm;
• (b) the step region having top and a bottom with a height of between 5 to 80 μm, and a substantially flat region of a length of between 30 to 500 μm extending from the nozzle, the either the top or bottom being substantially continuous with the top or bottom, respectively, of the nozzle and either the bottom or top being offset from the bottom or top, respectively, of the nozzle and substantially parallel with either the bottom or floor, or top or ceiling, respectively, of the nozzle, forming a step region configured for finite step emulsification;
• (c) a ramp region having an angle of between 10 and 80 degrees relative to either the floor or ceiling of the step region, the ramp region terminates in an imaging region having a height of 15 to 110 μm;
• and
• (d) the imaging region is configured to collect drops as a two-dimensional array and to provide for imaging and evaluation of the two-dimensional drop array.