Mikrokanallarda kanalın kavislilik oranının ve parçacıkların konsantrasyonlarının pasif parçacık ayrıştırması üzerine etkilerinin incelenmesi
Investigation of the effects of channel curvature ratio and particles' concentrations in curved microchannels for passive particle separation
Yazar: DENİZ İNCE
Danışman: PROF. DR. LEVENT TRABZON
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Mühendislik Bilimleri = Engineering Sciences

Özet:
Mikroakışkan sistemler, kolay imalat, ayarlanabilirlik, ucuzluk, hassasiyet ve basitlik özelliklerine bağlı olarak geleneksel teknolojilere göre üstünlükleri nedeniyle büyük ilgi görmüştür. Özellikle, bu sistemler kimya bilimi, mühendislik disiplinleri ve kliniklerinin yanı sıra LüÇ (Çip üstü Lab) ve μTAS (mikro-toplam-analiz sistemleri) uygulamaları alanında daha önemli hale gelmiştir. Bu sistemlerin amacı akış alanını kontrol ederek hücreleri yüksek oranlarda ayırma ve sıralama yeteneğine sahip bir mikroyapı oluşturmaktır. Bunu yaparak, mikroakışkan sistemler, kanser teşhisi, su saflaştırma ve sedimantasyon alanlarında parçacıkların heterojen bir karışımdan ayrıştırılması ve zenginleştirilmesi konusundaki başarısını göstermiştir.
Bu alanda yapılan çalışmaların çoğu, kan ve kanser hücrelerinin etkileşimini daha iyi anlamak adına sentetik parçacıklar üzerinde akış fiziğini incelemede yoğunlaşmıştır. Çeşitli deneylerin bir sonucu olarak, mikrokanallardaki parçacık-parçacık etkileşiminin, hidrodinamik parametrelerin yanı sıra, boyut, şekil ve uzunluk gibi birçok parametre ile bağlantılı olduğu kanıtlanmıştır. Bunlardan en önemlisi kuşkuşuz Dean Sürüklenme Kuvveti olmuştur. Kanallara kavislilik eklendiğinde ortaya çıkan bu kuvvetin de yardımıyla parçacıkları bir noktada odaklamak yani, kanal boyunca ip gibi dizmek mümkün hale gelmiştir. Fakat daha sonra yapılan çalışmalar Dean Sürüklenme kuvvetine ek olarak parçacıkların sahip olduğu Reynolds Sayısı ve λ sayısınında odaklanma mekanizmasında etkin olduğunu ileriye sürmüştür.
Bu çalışmada, parçacık-parçacık ayrıştırma mekanizması üzerinde herhangi bir formüle dâhil edilmeyen kanalların kavislilik derecesinin önemi ve parçacıkların başlangıç konsantrasyonlarının etkisi incelenmiştir. 3 farklı kanal geometrisinde başlanıp 9,9 µm parçacıkların odaklanma başarılarına göre nihai geometriye karar verilmiş sonrasında parçacık parçacık etkileşimi davranışı üzerinde konsantrasyonun etkisi detaylı bir şekilde incelenmiştir. Sonuç olarak, parçacık parçacık etkileşimininde odaklanma kriterlerini sağlayamayan parçacıkların diğer parçacıkların odaklanmasını olumsuz olarak etkilediği ortaya konmuştur. Deney sonuçlarına ek olarak Hesaplamalı akışkanlar dinamiği analizleri yapılmıştır. Analizlerin ve Deney sonuçlarının birbirleriyle uyum içinde olduğu gösterilmiştir.

Abstract:
Microfluidic systems have attracted a great deal of attention due to its upper hand over conventional technologies dependent on their easy fabrication, precise manipulation, cost-effectivity, sensitivity, and simplicity. Particularly, these systems have become more important in the field of chemical science, engineering disciplines and clinics as well as LoC (lab on a chip) and μTAS (micro-total-analysis-systems) applications. The aim of these systems is to create a microstructure which has the capability to control the overflow field thus separating and sorting cells at high rates. By doing so, microfluidic system manifests its achievement concerning fractionation and enrichment of particles from a heterogeneous mixture in the range scope of cancer diagnosis, water purification, and sedimentation.
Specifically, many scientists have utilized microsystems methodology to make an observation about a patient's health status. They enhanced methods to isolate circulating tumour cells called CTCs. Fundamentally, CTCs are known as rare cancer cells which come into existence from primary cancer cells and flow through the blood where first observed by Ashworth. Since a method isolating CTCs from blood cells called "liquid biopsy" found in the mid-90s, the topic has reflected it's potent to prognosis and effective treatment. To achieve effective treatment, biological cells and particles have been performed and further interpreted for the better understanding of the separation mechanism. This approach is thought to be a footprint to make an appropriate mechanism for isolating CTCs. 
Various particle-particle separation and focusing methods, which are classified as active and passive depending on the external force, have been working in the last few decades. Active methods necessitate external such forces as magnetic, dialectic, acoustic and optic. Remarkably, the active method is known to provide more accurate results but, adopting them into a device sustains its complexity. In other words, the requirement of expensive equipment and finding optimum parameters for separation phenomena restricts their ability for rapid separation. On the other hand, passive methods principally utilize hydrodynamic forces to separate cells. 
As mentioned above, the leading parameters in passive cell separation are the hydrodynamic forces as well as flow parameters. So as to identify the channels' geometric parameters such as size, shape, and length, many researchers have been working on different channels for further understanding the effect of channel variations on inertial focusing. Since channels differ one another relied on their focusing capability, they are classified into 4 basic categories dependent upon their shape: straight, curved, spiral, and serpentine. In straight channels, the equilibrium positions of the particles are slightly interlinked with hydrodynamic forces and hydraulic diameter of the channel. Moreover, pure equilibrium in straight channels is limited compared to curved channels, but some researchers have proved that well-designed principles have put frothed more effective designs. In contrast, curved channels have been proven their efficiency by introducing the tertiary force called Dean. The force originated from the dominant lift force at the channel center thus resulting in an increment force at the center by which fluid travels along inner wall to outer wall of the microchannel. This foundation causes two symmetrical vortices, which are perpendicular to flow direction, are in the top and bottom halves of the channel. In other words, this force induces two counter flow rotation at the channel centre which is responsible for the particles' movement in the transverse direction along the Dean vortices' velocity vectors, thus creating an equilibrium position in the channel cross-section. However, later studies have suggested that in addition to the Dean Drag Force, Reynolds Number and λ number of the particles affect the particle focusing ability. 
In this study, the objective is to build a Computational Fluid Dynamics model to observe the decoupling effects of fluid inertia with Dean Drag Force on the particle separation phenomena. It is also to probe deeply into the relationship between channel geometry and particle focusing ability with respect to a variety of flow rates and particles' concentrations. To realize these objectives, curvilinear channel geometries with the inner radius of 900 μm and curvature angle of 180°, 210° and 270° - respectively - were designed, which have not yet been investigated in the literature. After that, the focusing behaviors of 9.9 μm fluorescent polystyrene microparticles were examined solely at different flow rates in the channels. By deciding on the most effective channel geometry, the focusing behaviors of 1.1, 3.3 and 9.9 μm fluorescent polystyrene microparticles were investigated in detail. Consequently, the information gathered from this study shows that the effect of the curvature angle - which has not been mentioned in any formula - on the particle-particle separation, is nothing more than design criteria. In addition, increase of the curvature angle is found to have positive effect on focusing quality, thus resulting in a more efficient separation.  During the experiments, it is also seen that single focusing line has been observed on the particles that maintain the focusing criteria: and. On the other hand, the particles - not satisfying the focusing criteria – have deteriorated the focusing quality of the particles that meet the criteria. However, even If the focusing quality of the particles is diminished, they are still separable by achieving the appropriate concentration ratio. Additionally, both experimental and Computational Fluid Dynamics studies have been found to be in harmony.