Study On The Devices For The Breakup Of Droplets Based On Surface Acoustic Wave

Digital microfluids(droplets)are important manipulating objects in microfluidic systems because they are not only the carriers like containers loading or encapsulating(bio)chemical reactants such as DNA,porteins,cells,viruses,drugs or bioparticles etc,but also the microreactors where the biochemical reactions take place.The breakup of digital microfluids(droplets)is used not only for controlling the size of the droplets,but also for adjusting the droplet concentration.This technology is widely applied to DNA analysis,portein analysis,cell analysis,drug discover,bioparticle concentration and objective bioparticles separation.Piezoelectric devices are widely applied to microfluidic systems due to their simple technology and strong microfluidic operations ability.A microfluidic system on a piezoelectric substrate is an important branch of the microfluidic analysis system,and is paied attention to by scientists on microfluidic analysis.The breakup of droplets on the piezoelectric substrate is nessary operation for microfluidic system on the piezoelectric substrate.However,traditional method for splitting droplets needs to be imporved due to larger power of an electric signal applied to the interdigital transducer(IDT),which limits its application.Moreover,it is also difficult to adjust the volume ratio of split daughter droplets in the channels of present microfluidic systems.Here,the breakup methods for spliting droplets in channels or on the piezoelectric substrate have been presented,and three types of microfluidic devices have been designed and fabricated.The research results and main contributions of this dissertation are as follows:Firstly,a microfluidie deviee driven by surface acoustic wave on a piezoelectric substrate with a channel divided by a PVC thin slice is fabricated to solve the problem by adjusting volume ratio of split daughter droplets,which is referred as volume ratio controlled breakup device.An IDT is designed,and fabricated on 128°YX-LiNbO3 piezoelectric substrate using microelectronic technology.A polydimethylsiloxane(PDMS)channel is made by die casting,which is divided into two daughter channels by the PVC thin slice.The channel is mounted on the piezoelectric substrate.The position of the droplet to be split is adjusted by surface acoustic wave excited by the IDT.As soon as the droplet in continuous oil phase passes through the PVC thin slice,the droplet is split into two daughter droplets.A small PDMS choke plug is designed in one of the daughter channels for keeping the daughter droplets from re-fusion.Breakup experiments are carried out using red ink solution droplets in continuous paraffin oil phase.Results show that the volume ratios of the split daughter droplets are 0.92 and 1.43 at different power level SAW.Secondly,a new method of droplet breakup by suddenly decreasing the electric signal power is presented to solve the larger power of the electric signal owing to continuous SAW used in traditional droplet splitting,and thus the micropower breakup device is proposed.An interdigital transducer with 27.5 MHz center frequency is fabricated on a 128°YX-LiNbO3 piezoelectric substrate using microelectronic technology.The droplet on the piezoelectirc substrate will be deformated due to acoustic radiation force when an electric signal with certain power applied to the IDT.However,when acoustic streaming force suddenly disappears by moving away the electric signal,the upper part of the droplet will run away from the droplet due to the inertia.The lower part of the droplet keeps the original state by its gravity and surface tension.The dynamical property of the droplet is then analyzed.Red dye solution droplets are demonstrated as the experimental sample to verify the breakup.Results show that a 4?l droplet can be split several times when the electric signal power is suddenly decreased from 12.3 dBm to-3.98 dBm.Finally,a new method of splitting droplet on a piezoelectric substrate with precisely controlled vertical transportation height by help of a small ball is presented to satisfy mass charge tansportation among several substrates in complex biochemical analysis systems.Therefore,a vetical transportation breakup device is proposed.An interdigial transducer is fabricated on a 128°YX-LiNbO3 piezoelectric substrate using microelectronic technology.The droplet is driven and transported along the surface of the small ball by surface acoustic wave.A part of droplet on the top of the ball flies off at the condition of enough electric signal power.The mechanical porperty of the droplet is also described,and force conditions for vertically spatial transportion are given.The critical volume for successful transportion along the ball surface is deduced.A 5 ?l red ink solution droplet is successfully transported along the surface of the small ball with 2 mm diameter and then split at 30.8 dBm electric signal power.As applications,the physical mixture of two different droplets and color development reaction are sucessfully implemented in the homemade devices for splitting droplets.