This paper presents a study on the formation of dispersions using a "flow focusing" geometry in microchannels. The research is conducted in a microfluidic device with a flow-focusing configuration, which allows for the controlled formation of both monodisperse and polydisperse emulsions. The device uses soft lithography fabrication methods to create a planar microchannel. The study focuses on the formation of water droplets in an oil continuous phase, with the flow rates of the two liquids being varied to control the drop size and distribution. The flow-focusing geometry involves a liquid flowing into the middle channel and a second immiscible liquid flowing into the two outer channels. The two liquid phases are then forced through a small orifice, where the outer fluid exerts pressure and viscous stresses that force the inner fluid into a narrow thread, which subsequently breaks into droplets. The drop size is influenced by the flow rates and the ratio of the internal water flow rate to the external oil flow rate. The study reports a phase diagram illustrating the range of responses observed, showing different regimes of drop formation. In one regime, the drop size is comparable to the orifice width, while in another, the drop size is dictated by the diameter of a thin focused thread, resulting in drops much smaller than the orifice. The results show that the smallest droplets produced can be much smaller than the orifice radius, with drop sizes in the range of hundreds of nanometers. The study also notes that coalescence can occur when droplets collide downstream of the orifice, and that surfactants can be used to stabilize the droplets. The research highlights the potential of microfluidic devices for the controlled formation of emulsions and dispersions, with applications in various fields such as personal care products, food, and drug delivery. The study also discusses the importance of wetting issues in small devices, as highlighted by recent research.This paper presents a study on the formation of dispersions using a "flow focusing" geometry in microchannels. The research is conducted in a microfluidic device with a flow-focusing configuration, which allows for the controlled formation of both monodisperse and polydisperse emulsions. The device uses soft lithography fabrication methods to create a planar microchannel. The study focuses on the formation of water droplets in an oil continuous phase, with the flow rates of the two liquids being varied to control the drop size and distribution. The flow-focusing geometry involves a liquid flowing into the middle channel and a second immiscible liquid flowing into the two outer channels. The two liquid phases are then forced through a small orifice, where the outer fluid exerts pressure and viscous stresses that force the inner fluid into a narrow thread, which subsequently breaks into droplets. The drop size is influenced by the flow rates and the ratio of the internal water flow rate to the external oil flow rate. The study reports a phase diagram illustrating the range of responses observed, showing different regimes of drop formation. In one regime, the drop size is comparable to the orifice width, while in another, the drop size is dictated by the diameter of a thin focused thread, resulting in drops much smaller than the orifice. The results show that the smallest droplets produced can be much smaller than the orifice radius, with drop sizes in the range of hundreds of nanometers. The study also notes that coalescence can occur when droplets collide downstream of the orifice, and that surfactants can be used to stabilize the droplets. The research highlights the potential of microfluidic devices for the controlled formation of emulsions and dispersions, with applications in various fields such as personal care products, food, and drug delivery. The study also discusses the importance of wetting issues in small devices, as highlighted by recent research.