Study On Elastic Instability Of Microscaled Extensional Viscoelastic Fluid Under External Excitations

Microfluidic technology is widely applied in the fields of chemical analysis,medical diagnosis and biological science and so on.For being used as a transportation medium,the Non-Newtonian microfluid is usually used such as tissue fluid in the biological experiments and macromolecule polymer in the chemical analysis.As a typical viscoelastic fluid,the macromolecule polyacrylamide(PAM)solution has totally different flow properties with that of Newtonian fluid.Due to its internal significant elastic stress under microscale,the PAM manifests special elastic instabilities such as flow rate oscillation with time or flow field asymmetry under external excitations or extensional flow effect.Therefore,the research of the instabilities of viscoelastic fluid under microscale is not only meaningful to real applications but also provide experiment references for the fundamental study of the viscoelastic effect under microscale.In this thesis,the instabilities of viscoelastic fluid under different boundary conditions are studied both numerically and experimentally.First,microfluidic chip with micrometer wide channel is fabricated for the experimental investigation of viscoelastic flow in microscale.Conventional lithography technology and softmater fabrication technology are applied.A control system for the microfluidics is also well established.Meanwhile,the fluid in the experiment is systematically characterized through both classical rotational rheometer and microrheological method.Both Newtonian fluid and Non-Newtonian fluid,here glycerol and PAM,are characterized.The measured results through the conventional rheometer in the low frequency regime and through the microrheological method in the high frequency regime compensates each other.In the conventional rheometer,the fluid flow rate and pressure is measured and the viscosity and elasticity of the fluid is derived.Furthermore,special viscoelastic properties such as the shear thinning effect and elasticity are also obtained qualitatively.Next,a numerical solver based on the Oldroyd-B model built on the open source platform OpenFOAM is development to analyze the viscoelastic flow with high Weissenberg number.External body force is introduced to drive the viscoelastic flow.The temporal instable response of the viscoelastic fluid under external forces in the Poiseuille flow condition is numerically investigated.The amplitude of external forces,force loading time and force oscillation period are varied to manipulate the fluid instability.The resulted instable flow rate is fitted for different initial and boundary conditions.An underdamped oscillation is induced in the flow rate response under a constant driving force.The “platform effect” is raised by the rectangular driving force and occurs at the first 1/4 period of the flow oscillation.Fourier transform is applied to the time changing flow rate and the resonant frequency of the flow is investigated.Flow becomes resonant when the external force driving frequency matches the viscoelastic fluid intrinsic frequency,and the frequency response of the flow rate is single resonant at a high amplitude.This phenomenon can be applied for the derivation of the viscoelastic fluid relaxation time.Furthermore,the viscoelastic instabilities are experimentally manipulated in different designed microchannels.Extensional flow is usually induced and flow velocity gradient is thus generated,which exerts a stretching and relaxation effect to the polymers in the viscoelastic fluid.As a result,the fluid instability is raised.It is founded experimentally that the flow symmetric stable state,asymmetric bistable state,and unstable state can be induced in the fluid with different Weissenberg number and different structured microchannels.The symmetry property and flow bias are therefore effectively manipulated.In the last,the mixing behavior of viscoelastic fluid in a T-shaped microfluidic channel with abrupt expansion structure and external excitation is studied,the mixing degree is defined and characterized.It is founded that the mixing is suppressed in the Newtonian fluid while enhanced in the viscoelastic fluid at the high flow rate.Meanwhile,the influence of different boundary conditions and different T-structure shapes on the instability and mixing are investigated.Based on the standard T-shape microchannel structure,different channel structures are designed such as a long-neck T-structure or wide mixing channel T-structure.It is founded that in the long-neck Tstructure a more uniform mixing can be obtained,which relies on the impact of the prestretch of the viscoelastic fluid in the long-neck channel.While for the excitation at double inlets,the mixing degree depends much on the excitation amplitude,and is enhanced as the two excitations are out of phase.In a summary,the instability of viscoelastic fluid in the microscale condition are both numerically calculated and experimentally measured.The instabilities under different excitations and in different shaped microchannel structures are investigated,which shows that the flow instability can be induced and manipulated effectively.Based on the viscoelastic instability control,different microfluidic devices such as micromixer,fluid memory and transducers can be realized.