Active control of microdroplets.
ABSTRACT In this project, the formation of ferrofluid droplets in T-junction and flow focusing microfluidics and the effects of magnetism on the formation of the ferrofluid droplets were demonstrated. The chips were fabricated using PDMS, both oil based and water based ferrofluids were used in the formation of droplets, while mineral oil, silicon oil 50cSt and silicon oil 100cSt were used as the continuous phase. The imaging technique consists of an illumination system, an inverted microscope (Model ECLIPSE TE2000-E), a coupled charge device (CCD) camera and a control system. The control system consists of a LabView program implemented in a desktop computer, which was used to take image sequence of the droplet formation process. In this project, water based ferrofluid was used to form droplets in a flow focusing device. Ferrofluid droplets were able to be formed using water based ferrofluids with the continuous phase as mineral oil, silicon oil 50cSt and silicon oil 100cSt. A magnet attached to a needle was pierced into the microfluidic chip, as an introduction of magnetism to the process. With the magnetized needle at the disperse phase channel of the microfluidic device, the droplet sizes seem to be reduced compared to those measured without magnetism effect. The addition of the needle drastically reduced the magnetic strength, and was therefore removed. Water based ferrofluid droplets were formed in the T-junction device. The ferrofluid droplet diameter seems to decrease linearly as the continuous phase flow rate was increased. A permanent magnet was placed at fixed horizontal distances from the disperse phase channel in the upstream and downstream locations. The graphs of the experimental data show that the droplet diameter size increases and the magnet is placed in the upstream location of the T-junction, while the droplet diameter size decreases as the magnet is placed in the downstream location of the T-junction. At about a 3500µm away from the centre of the magnet, the magnetism seems to have little effect on the droplet formation. An experiment at a higher flow rate of continuous phase flow rate was done, and a similar trend was spotted, although the droplet sizes did not vary as much as the lower flow rate. Using an axial probe, the change in magnetic field direction can be seen, with the field strength from the positive changing to negative value. While using the transverse probe, the reduction in magnetic strength with increasing distance away from the centre of the magnet can be seen. This is the cause that the variation in ferrofluid droplet size is reduced as the permanent magnet is placed further away from the disperse phase channel. This proves the trend in the droplet size versus magnet distance graph. MECHANICAL ENGINEERING