Methods and devices are provided for manipulating droplets on a support using surface tension properties, moving the droplets along a predetermined path and merging two droplets together enabling a number of chemical reactions. Disclosed are methods for controlling the droplets volumes. Disclosed are methods and devices for synthesizing at least one oligonucleotide having a predefined sequence. Disclosed are methods and devices for synthesizing and/or assembling at least one polynucleotide product having a predefined sequence from a plurality of different oligonucleotides having a predefined sequence. In exemplary embodiments, the methods involve synthesis and/or amplification of different oligonucleotides immobilized on a solid support, release of synthesized/amplified oligonucleotides in solution to form droplets, recognition and removal of error-containing oligonucleotides, moving or combining two droplets to allow hybridization and/or ligation between two different oligonucleotides, and further chain extension reaction following hybridization and/or ligation to hierarchically generate desired length of polynucleotide products.