GRADUATE SCHOOL OF SCIENCES & ENGINEERING
ELECTRICAL AND ELECTRONICS ENGINEERING
PhD THESIS DEFENSE BY M. ZEESHAN RASHID
Title: Fabrication of surface-assisted liquid manipulation system
Speaker: M. Zeeshan Rashid
Time: 13/08/2018, 12:00
Place: ENG 208
Rumeli Feneri Yolu
Thesis Committee Members:
Prof. Dr. Alper Kiraz (Advisor, Koç University)
Prof. Dr. Alper Erdoğan (Koç University)
Prof. Dr. Metin Muradoğlu (Koç University)
Asst. Prof. Dr. Kenan Çiçek (Iğdir University)
Asst. Prof. Dr. Ahmet Erten (Istanbul Technical University)
In this thesis, controlled motion of various polar liquids along patterned tracks defined over certain polymer surfaces is demonstrated. The liquid motion along the tracks is maintained due to their relatively hydrophilic (water attractive) nature as compared to the surrounding hydrophobic (water repellent) region. As a result, liquid, either in the from of droplets or bulk, is driven along the tracks in the direction of external pressure ow. Such polymer microfluidic devices are fabricated using polydimethylsiloxane (PDMS) which allows easy handling, artificial roughness addition, reconfigurability, non-toxic nature and sensitivity to certain external stimulus. Fabrication of reconfigurable polymer surfaces are demonstrated which switch their wettability from superhydrophobic to superhydrophilic upon exposure to oxygen plasma and return to their original state after appropriate thermal treatment for many cycles. This is a purely chemical process taking place due to the migration of functional groups from surface to the bulk and vice versa which causes dramatic transition of wetting state and also allows absolute recovery of surface hydrophobicity. Wetting properties of these surfaces are determined by measuring their static, advancing and receding water contact angle. A specific PDMS mask with narrow openings is developed which allows selective exposure to the surface to make hydrophilic channels. Over these channels, water filaments are produced due to surface tension driven transport phenomenon from one side called input reservoir to the other side, output reservoir. Those patterns are absolutely erasable by thermal treatment, so that, new patterns can be made in the same way for many times.
Hydrophilic tracks can also be defined by exposing PDMS-coated glass slides via laser ablation which is an irreversible process and only changes the topography. Laser ablation is a process of exposure of a high power extremely focused laser beam to remove PDMS coating, thus, uncovering the glass substrate. Since the wettability contrast is not too much, instead of surface driven transport, this protocol is adopted to manipulate droplets of nanoliter volume immersed in oil in a closed microfluidic environment. The difference in wettability of glass and PDMS surfaces together with the shallow step-like transverse topographical profile of the ablated tracks allow polar droplets wetting preferentially the glass surface to follow the track. Droplets with smaller wettability are found to be unguided even in the presence of topographic step. Based on this phenomenon, passive sorting of microdroplets of different chemistry injected in the same microfluidic chip is also explored. A comprehensive experimental and theoretical study is done with different droplet liquids, ow speeds and geometry of guiding tracks to gain deep and detailed understanding of the system.
Liquid filaments over patterned surface are of paramount importance in photonics for fabricating fluidic optical waveguides. Guiding of light through these liquid waveguides can benefit bio-sensing, bio-lasing mostly within the visible spectrum in a highly efficient way. The theoretical implementation of an optofluidic rhodamine B dye laser based on holey fiber is demonstrated in the last chapter of the thesis. Conventional dye laser is also modelled for a comparison with the fiber laser. It is concluded that a specific fiber based variant of dye laser is superior to the conventional dye laser for having smaller lasing threshold and higher slope efficiency. These types of fluidic lasers can also be established by fluidic waveguiding over reconfigurable patterned surfaces.