Varanakkottu, Subramanyan Namboodiri
Light-Induced Microfluidic Transport Phenomena.
TU Darmstadt, Darmstadt
[Ph.D. Thesis], (2013)
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|Item Type:||Ph.D. Thesis|
|Title:||Light-Induced Microfluidic Transport Phenomena|
Optofluidics is an emerging field which combines microfluidics and optics, having widespread applications in fundamental sciences as well as engineering. Among the research in the area of optofluidics, manipulation of small objects such as particles and droplets is of great interest. Precise control over the manipulation and confinement of such objects is a challenging task. Unification of microfluidics and optics opens a new way to achieve this goal with added advantages such as non-contact manipulation capability and tunability. This Ph.D. dissertation addresses optofluidic manipulation of particles and droplets based on some novel concepts.
The section on light-induced particle manipulation begins with optical trapping inside a microfluidic channel. Motivation of this study is to understand the influence of velocity profile on the trapped particle. An optical trapping experimental setup is constructed using a He-Cd laser (442 nm emission) as the trapping source. Optical trapping experiments are performed under two flow conditions. A particle trapped inside a microfluidic channel experiences a parabolic velocity profile. In the second method, particles are trapped inside a sample chamber where the trapped particle experiences a uniform velocity profile. Experiments are performed at different optical powers with particles having various diameters. Results showed that for particles having intermediate size the trapping force is higher in the case of particles trapped inside the microfluidic channel than that of a sample chamber. This is attributed to the contribution of Saffman lift force arising from the parabolic velocity gradient. Experimentally measured optical trapping stiffness is found to be in good agreement with the theoretical model.
Following that, a novel particle manipulation technique is presented. Here, microparticle adsorbed at the air-water interface is trapped and manipulated along the interface. The method relies on photoresponsive surfactants adsorbed to a gas-liquid interface that can be reversibly switched between two isomeric states (a trans state and a cis state) using light beams. The principle is based on local changes of the surface tension, giving rise to Marangoni stresses. Depending on the type of surfactant isomer in the region around the laser spot, a flow either radially inward or outward is created. For the trapping of microparticles, a 325 nm beam from a He-Cd laser is focused at the interface, which results in an inward flow directing towards the focal spot. This inward flow is utilized for trapping and manipulation of particles. Interfacial flow velocity is characterized using particle streak velocimetry. It is experimentally demonstrated that this trapping mechanism is capable of manipulating the trapped particle at lower intensity than conventional optical tweezers.
Finally, studies on light-induced droplet manipulation were conducted, utilizing the phase transition of temperature sensitive PNIPAM (Poly(N-isopropylacrylamide)) polymer films. PNIPAM films are prepared on UV absorbing glass plates. Absorption of the UV light by the glass raises its temperature resulting in the phase transition of PNIPAM film from a swollen (hydrophilic) to a deswollen (hydrophobic) phase. Experiments show that PNIPAM films undergo a phase transition from a hydrophilic to a hydrophobic state at around 26oC. At the hydrophobic state, water drop placed on the substrate exhibits a contact angle of about 78o while it reduces to 53o at the hydrophilic state. Experiments are performed to drive the water drop by creating a wettability gradient over the surface by locally cooling one side of the drop. Though the drop spreads towards the colder region, due to the large hysteresis in contact angle, the receding edge of the drop is pinned at the surface.
|Place of Publication:||Darmstadt|
|Uncontrolled Keywords:||Optofluidics, Interfaces, Optical tweezers, Marangoni flow, Wettability|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 500 Naturwissenschaften|
|Divisions:||16 Department of Mechanical Engineering|
|Date Deposited:||09 Jul 2013 15:13|
|Last Modified:||09 Jul 2013 15:13|
|Referees:||Hardt, Prof. Steffen and Dreizler, Prof. Andreas|
|Refereed:||28 May 2013|