טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAlicia Boymelgreen
SubjectSymmetry Breaking in Non-Linear Electrokinetic Colloidal
Transport at the Micro/Nanoscale
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Yossifon Gilad
Full Professor Touvia Miloh


Abstract

One of the primary initial challenges in the development of microfluidic systems was the search for an appropriate forcing mechanism. Recognizing that silicon technology, already well-developed for microelectronic applications, could be adapted to both build microfluidic devices and integrate microelectrodes within them spurred the advancement of electrokinetically driven microfluidics. Scaling with the typical length scale squared (as opposed to pressure driven flow for example, which scales according to ), electric fields were shown capable of precisely and efficiently manipulating fluids at the micro (and nano) scale, with the added advantage of having no moving parts.

Within the general category of electrokinetic forcing, AC fields maintain a unique position with advantages including potentially reduced applied voltages and Faradaic reactions. However, since time-averaged forcing in an AC signal is zero, net motion can only arise from effects which are non-linear in nature. Moreover, for most practical applications, such as pumping or propulsion of colloids it is necessary to also introduce asymmetry into the system - either through the geometry or the electric field. This combination of non-linearity and symmetry-breaking can significantly complicate system characterization and especially impedes the development of comprehensive theoretical models and numerical simulations which are critical for optimization.

Within this work, we focus on two important nonlinear mechanisms, which govern the mobility of symmetry-broken Janus spheres subject to AC electric fields; dielectrophoresis1 and induced-charge electroosmosis2 - which together can be combined under the umbrella term of dipolophoresis3. Electrokinetically driven Janus particles represent a unique platform, lying at the intersection of the traditional fields of self-propulsion and phoresis, since although the particles will only move in the presence of an externally applied field (similar to phoretically driven colloids), their direction of motion is not restricted by this field and they are free to translate along individual pathlines and also exhibit group behavior4. The key to this apparent paradox is that despite the presence of an external applied field, the gradients actually driving the motion are produced on the particle level rather than over the entire system. As such, the Janus particles may best be classified as active colloids5.

Within this body of work, we aim to rigorously characterize the frequency response of these particles, explaining its unique features via a comprehensive examination of all relevant physical mechanism occurring within the system as a whole. Accordingly, we assume a two pronged approach - complimenting measurement of the frequency dependent velocity of freely suspended Janus particles with the observation of hydrodynamic flow around stagnant colloids. One of the most significant results is the revelation of a novel propulsion mechanism, termed “self-dielectrophoresis”, wherein at high frequencies the Janus particles reverse direction to travel with their metallic hemisphere forward as the result of an electrostatic (dielectrophoretic) force. Ultimately, it is intended that this characterization will aid in the implementation of Janus particles in practical systems and we conclude by demonstrating two, novel applications; a microfluidic mixer and an all in one cargo carrier.