Dynamic manipulation by light and electric fields: Colloidal particles to droplets

The realm of microfluidics deals with the study, control and manipulation of fluids that are geometrically constrained to small, typically sub-micrometer length scales. Many microfluidic applications require non-invasive manipulation of fluid or even particulates suspended in a fluid medium. For continuous microfluidic systems, non-invasive manipulation of fluid may be required by pumping or mixing applications, while in-situ concentration or sorting or manipulation of particles, cells, viruses or molecules is an operation that is often necessitated by sample reaction and detection processes in lab-on-a-chip devices. On the other hand, digital microfluidics, with its 'digitization' of liquid streams into discrete droplets, requires non-invasive manipulation of liquid droplets thus allowing diverse biomedical applications to be parsed into easily realizable elemental fluidic 'building blocks'.;Hybrid opto-electric techniques (HOTs) for non-invasive manipulation of matter over several length scales are developed. Rapid electrokinetic patterning (REP) is one such HOT for the non-invasive manipulation of micro and nano-particles on an electrode surface. The technique employed a simple parallel electrode setup with an applied AC electric field on which optical holograms at a suitable wavelength are shone. In a low AC frequency regime (<200 kHz), particles could be dynamically configured into various shapes by the use of holograms generated from spatial light modulator (SLM). REP is primarily an electrokinetic technique, where optically modulated electrokinetics serves to rapidly transport particles to the illuminated locations on the electrode surface leading to particle aggregation. It is shown that a particle transport to the illuminated location is enabled by a toroidal electrothermal microvortex (EMV). Two-dimensional micro particle image velocimetry (muPIV) measurements were performed on the EMV flow. Further, we study the three dimensional nature of the toroidal EMV by using wavefront deformation particle tracking velocimetry (PTV) developed at Universitat der Bundeswehr, Germany. By using a cylindrical lens in conjunction with a microscope objective lens, wavefront deformations are introduced. The tracer particles appear as ellipses and this elliptical nature of the image encodes out-of-plane information regarding the particles' position. Numerical simulations agree well with experimental measurements. REP is a promising new tool for non-invasive manipulation and concentration. While many different applications of REP can be conceived, two applications of this newly discovered technique are showcased -- force measurement and sorting of particles.;Further development led to the creation of a universal hybrid opto-electric platform (u-HOP). u-HOP leverages a variety of different physical mechanisms to achieve dynamic manipulation of droplets and also achieve in-situ concentration of colloidal particles suspended in the droplets. Various physical mechanisms are leveraged through different modes of operation of the device. Each operational mode, which is activated through the proper combination of an applied AC bias and the illumination used, is characterized by the ability to manipulate objects at a certain length scale. It is also demonstrated that the device lends itself towards the 'reactive control' of microstructure patterns that emerge from a droplet evaporation process.