A method of controlling the formation of droplets of a droplet fluid composition from a jet of the droplet fluid composition, the method comprising: providing a microfluidic device having at least one channel for the passage of the droplet fluid composition leading via an orifice to a droplet receiving space, the orifice having a width, the channel having a width that is greater than the width of the orifice; providing a bluff body positioned within the channel, the bluff body having a geometry; providing a flow of the droplet fluid composition through the channel and around the bluff body, emanating as a fluid jet from the orifice, the fluid jet emanating from the orifice having a characteristic Raleigh frequency, which determines the natural frequency of jet break up in the droplet receiving space, characterized by: passively creating a boundary layer separation of the flow of the droplet fluid composition as the flow of droplet fluid composition flows around the bluff body located within the channel using the width of the channel, the width of the orifice, and the geometry of the bluff body, the boundary layer separation forming a periodic flow instability in the flow of droplet fluid composition as the flow of droplet fluid composition flows around the bluff body located within the channel, the periodic flow instability of the fluid flow having a frequency for the width of the channel, width of the orifice, and that geometry of the bluff body that is within an order of magnitude of the Rayleigh frequency for the jet of the fluid emanating from the orifice; and controlling the formation of droplets in the fluid composition from the jet of the fluid emanating from the orifice into the droplet receiving space by perturbing the fluid jet using the periodic flow instability. providing a microfluidic device having at least one channel for the passage of the droplet fluid composition leading via an orifice to a droplet receiving space, the orifice having a width, the channel having a width that is greater than the width of the orifice; providing a bluff body positioned within the channel, the bluff body having a geometry; providing a flow of the droplet fluid composition through the channel and around the bluff body, emanating as a fluid jet from the orifice, the fluid jet emanating from the orifice having a characteristic Raleigh frequency, which determines the natural frequency of jet break up in the droplet receiving space, characterized by: passively creating a boundary layer separation of the flow of the droplet fluid composition as the flow of droplet fluid composition flows around the bluff body located within the channel using the width of the channel, the width of the orifice, and the geometry of the bluff body, the boundary layer separation forming a periodic flow instability in the flow of droplet fluid composition as the flow of droplet fluid composition flows around the bluff body located within the channel, the periodic flow instability of the fluid flow having a frequency for the width of the channel, width of the orifice, and that geometry of the bluff body that is within an order of magnitude of the Rayleigh frequency for the jet of the fluid emanating from the orifice; and controlling the formation of droplets in the fluid composition from the jet of the fluid emanating from the orifice into the droplet receiving space by perturbing the fluid jet using the periodic flow instability. passively creating a boundary layer separation of the flow of the droplet fluid composition as the flow of droplet fluid composition flows around the bluff body located within the channel using the width of the channel, the width of the orifice, and the geometry of the bluff body, the boundary layer separation forming a periodic flow instability in the flow of droplet fluid composition as the flow of droplet fluid composition flows around the bluff body located within the channel, the periodic flow instability of the fluid flow having a frequency for the width of the channel, width of the orifice, and that geometry of the bluff body that is within an order of magnitude of the Rayleigh frequency for the jet of the fluid emanating from the orifice; and controlling the formation of droplets in the fluid composition from the jet of the fluid emanating from the orifice into the droplet receiving space by perturbing the fluid jet using the periodic flow instability.