Dispersed droplets and bubbles were generated in a microfluidic flow-focusing device to investigate the morphological developments of the droplets (bubbles) flowing in the downstream straight microchannel by a high speed camera. The present study is mainly aimed at the influences of both the dispersed droplet (bubble) size and two-phase average flow velocity on the formation of Lip stream in the rear of a droplet (bubble). It was found that the particle deformation increased with the droplet (bubble) length or the capillary number. There exists a critical droplet (bubble) length dependent on capillary number, beyond which the tip stream ejecting tiny daughter droplets will take place. The generated tiny droplets are usually three orders of magnitude smaller than primary droplet. This study provides the possibility to avoid a high polydispersity index in microchannels. (C) 2015 Elsevier Ltd. All rights reserved.