Microfluidic Devices: Effect of Elasticity and Surfactants on Drop Deformation and Break Up

University Of New Hampshire, Durham NH

Investigators

Abstract

ABSTRACT - 0457557 University of New Hampshire The goal of this project is to advance the understanding of two phase viscoelastic flows in microfluidic devices and to develop means to engineer new microdevices. Tremendous progress has been made in the development of microfluidic devices in the last decade with applications such as a "lab-on-a-chip", ink-jet printer heads, cell sorters, DNA sequencers, and blood-testing systems. Microdroplets are being used in microfluidic devices as actuators, chemical microreactors, drug delivery devices, and bioterror detection tools. However, a limited number of experimental studies exist to date on the deformation and break up of drops and bubbles in microfluidic devices. Typical applications of two-phase systems in microdevices involve viscoelastic fluids such as DNA solutions, blood, serum, and inks and suspensions with surfactants added as stabilizers. The stretching of polymer chains present in viscoelastic fluids causes large normal stresses to develop at strongly deforming interfaces affecting the dynamics of drops. Surfactant molecules adsorbed at the interface can redistribute along the interface leading to a local reduction in surface tension. This may result in highly curved interfaces and reduced tangential velocities due to the induced Marangoni stresses. The dynamics of a fluid particle in the presence of elastic and surfactant effects can be very complex and has been studied to a very limited extent even at a macroscale. The major research objectives of this study are: (1) to identify parameters governing the drop size and distribution of drops forming in a rectangular microchannel; (2) to determine the drop velocity and deformation for a drop translating in a rectangular microchannel; (3) to determine the effect of elastic stresses on the drop size and distribution in the drop formation and terminal velocity of a drop translating in a microchannel, and (4) determine the effect of mass transfer kinetics of surfactants on the deformation and pinch-off dynamics of drops in the microchannel. Intellectual Merit: The research objectives will be accomplished by performing a comprehensive experimental and numerical study of viscoelastic drops and bubbles in microdevices in the presence of surfactants. Microfluidic devices will be fabricated using soft lithography techniques in the PI's research laboratory. Two classes of experiments will be completed: (a) the deformation of a single drop/bubble translating in a straight channel in the presence of surfactants and/or elasticity and (b) the deformation and subsequent break up of viscoelastic drops/bubbles generated in a microdroplet generator with surfactants. A three-dimensional numerical model will be developed to predict the shape and velocity of a drop forming or translating in a microchannel using a hybrid volume-of-fluid method. The front-tracking scheme which explicitly tracks the interface predicts the interfacial topography and stresses accurately allowing simulations of drops with low density and viscosity ratios. The ability to predict the shapes and distributions of viscoelastic drops in microfluidic devices opens up exciting possibilities for the development of novel microdevices which use two-phase viscoelastic biofluids, suspensions, and polymer blends. Broader Impact: In addition to advancing the field of interfacial phenomena in microfluidic devices, this research will lead to a new elective course in the Chemical Engineering undergraduate/graduate curriculum on 'Microfluidics: Theory and Applications'. The course will draw material from the PI's own research and will provide the students with hands-on experience in her laboratory. This project will create the first facility for soft lithography at the University of New Hampshire. The PI, being a woman and a minority engineer herself, will encourage the involvement of women and members of minority groups in her research and in the field of science and engineering in general.