Microscale devices and reactions in microscale devices

A method for merging microdroplets comprising:

• a) providing first and second liquid microdroplets each having volumes between 1 and 100 nanoliters, a liquid microdroplet delivering means, and a device, said device comprising:
• i) a housing comprised of silicon,
• ii) first and second microdroplet transport channels etched in said silicon and connecting to form a third transport channel containing a reaction region, said first, second and third transport channels the channels having a depth between 0.35 and 50 .mu.m and a width between 50 and 1000 .mu.m,
• iii) a microdroplet receiving means in liquid communication with said reaction region via said transport channels, and
• iv) microdroplet flow-directing means comprising a series of heating elements arrayed along said first, second and third transport channels;
• b) delivering said first liquid microdroplet via said microdroplet delivering means to said first transport channel;
• c) delivering said second liquid microdroplet via said microdroplet delivering means to said second transport channel;
• and
• d) conveying said microdroplets in said transport channels to said reaction region in said third transport channel via said microdroplet flow-directing means by differential heating of said microdroplets by said heating elements, thereby merging said first and second microdroplets to create a merged microdroplet.