The Microfluidic Chip Based On The Photorefraction Of Lithium Niobate Crystal

The microfluidic chip based on the photorefraction of the lithium niobate（LN）crystal is regarded as a potential platform for biomedical applications such as biological analysis,clinical diagnostics and drug discovery.The photorefractive properties of LN make possible the photo-assisted dielectric microdroplet manipulation on chips where electrode fabrication isn’t required.However,no work was reported on manipulating aqueous microdroplets so far.We find that the DEP force can be utilized to actuate the non-polar dielectric liquids on the LN-based microfluidic chips,but it cannot directly drive water microdroplets well.Such a result is associated with the high permittivity and conductivity of water:under the photorefractive field,there will be many polarization and free charges produced in water microdroplets,and they can screen the photovoltaic charge distribution and reduce significantly the static DEP interaction from the LN surface.In order to analysis the photorefractive characteristics of LiNbO₃:Fe crystals,we systematically study the dynamic angular distribution of photoinduced light scattering（PILS）of the LiNbO₃:Fe crystals with the different Li compositions,and find that the Li-deficient congruent crystal is suitable for the chip substrate material.According to the dielectric wetting effect,we design the LN-based microfluidic chip and build the experimental setup for the aqueous microdroplet manipulation.Secondly,we demonstrate a successful photovoltaic manipulation of water microdroplets on the c-cut LN substrates coated with a Teflon film.Through the analysis of the dynamic process of the photovoltaic manipulation and the simulation of the electrostatic distribution around the microdroplet-substrate interface,both the hydrophobicity and the large electrostatic-energy capability of the Teflon film are emphasized for the manipulation of the water microdroplet:under the non-uniform electric field,there will be different voltage drops across the dielectric layer,leading to a asymmetrical change of the droplet contact angle and consequently to the movement of the water microdroplet.In addition,we exclude the connection of the microdroplet movement with the photo-induced thermal effect which is also utilized in our work for continuous generation and simultaneous transfer of the dielectric droplets.Finally,we study in this thesis the water microdroplet manipulation on the y-cut LN-based microfluidic chip,and find that the microdroplet can’t be manipulated smoothly along the+c dirction on the chip.Through the simulation,the difference of the charge diffusions between–c and+c directions is suggested to account for this phenomenon.And the PILS results show that the scattering light mainly distributed in the-c direction of the LN crystal,i.e.the photorefractive effect in the-c direction is relatively larger than+c direction.We confirm that the different phenomena of the water microdroplet manipulation between the–c and+c directions on the y-cut LN chip are related to the diffusion field directly.In addition,the non-local transportation of the water microdroplet is realized on the y-cut LN-based microfluidic chip by using the background light field and the trigger light field synthetically.