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Topological Defects in Active Nematics
, M.Sc. Thesis Sharif University of Technology ; Ejtehadi, Mohammad Reza (Supervisor)
Abstract
Topological defects are specific signatures of liquid crystals. Their presence in liquid crystals are so often that they are ”vestiges” of such flows. With their presence inside a flow, they cause global change to the director field by dictating specific orintational restrictions. So their global effects on the flow are substantial. In active nematics, not only they dictate the global structure of the director, but also they act as local sources of motion and behave like self-propelled particles. In this thesis, a detailed dynamics of active liquid crystals and their topological defects is presented. Then, Solving equations of motion for such systems using computer numerical analysis, the...
Gaussian Theory for Derivation of Continuum Equations of Self-propelled Particles
, Ph.D. Dissertation Sharif University of Technology ; Ejtehadi, Mohammad Reza (Supervisor) ; Moghimi, Saman (Co-Advisor)
Abstract
The collective behavior of active matters, e.g. colony of micro swimmers and flocks of birds is modeled with self-propelled particles. It is evident that a continuum description of such systems is useful in determining the collective behavior in large scales. One can make continuum equations in active matter with the help of symmetry arguments. However, the equation is in a phenomenological level with undetermined transport coefficients. It is possible to construct the continuum equations from microscopic rules to find the transport coefficients in terms of microscopic parameters with approximations. One of the usual approximations called truncation method is to truncate the Fourier series...
Flocking Behavior of Self Propelled Particles Under Confinement
, Ph.D. Dissertation Sharif University of Technology ; Ejtehadi, Mohammad Reza (Supervisor) ; Seyed Reihani, Nader (Co-Advisor)
Abstract
Studying the interaction of active particles with walls and obstacles has a great importance for practical purposes. Using this knowledge, it is possible to control the motion of active particles in different environments.Experimental evidences show that after encountering with a wall, most of the active particles slide along the wall for some time and then detach in a certain direction, independent of their incoming angle. This special collision is considered to guide, sort or trap active particles.In this dissertation, using Brownian dynamics simulations and theories, we study how different selfpropelled particles and walls influence each other. We explore the behavior of two important...
Predator Effects on the Stability of Collective Motions of Self-Propelled Particles
, M.Sc. Thesis Sharif University of Technology ; Jalali, Mir Abbas (Supervisor)
Abstract
The design and control of collective robots for different purposes has been one of the most interesting subjects in recent years. Interacting particles (both in engineering and nature) can not only be self-organized, but also show a myriad of amazing features. A lot of different types of collective motion can be seen in nature (like fishes, birds, bacteria, insects, to name but a few) which interacting with each other to form complicated patterns with ingenious purpose like foraging, predator escaping and so forth. Understanding these natural phenomena (like vortex formation) can be helpful in designing artificial systems, the usage of which are underwater or space vehicles performing a...
Numerical Simulation of Self-propelled Droplet over a Surface with Surface Tension Gradient
, M.Sc. Thesis Sharif University of Technology ; Javadi, Khodayar (Supervisor)
Abstract
Ice accretion may cause malfunction or serious performance degradation in outdoor facilities and structures, such as aircraft, ship, locks and dams, offshore platforms, solar panels, wind turbines, power transmission towers and lines, and sports facilities, leading to huge economic loss or even loss of human lives. Icephobic materials, typically applied in the form of coatings, have received growing attention in the last decade. This work focuses on surfaces with a surface tension gradient which has icephobic properties. various surface tension gradients such as linear, exponential, cycloid and parabola have been imposed on a surface in order to achieve the shortest time for a droplet to...
Analysis of Motility Phases of Binary Active Matter
, M.Sc. Thesis Sharif University of Technology ; Ejtehadi, Mohammad Reza (Supervisor)
Abstract
The study of active matter has gathered a great deal of attention in the past decades. The collective behavior of living matter such as the murmuration of the flocks of birds is an example of these systems. In order to study different aspects of these systems, variant models have been studied and change of behavior, phase shift and the existence of multiple phases has been observed.Vicsek model was one of the first models studying these systems, where an aligning pairwise interaction causes the agents to move in the same direction.We investigated Vicsek model by considering an overdamped Langevin dynamics for fewparticle systems. We showed Vicsek model in fewparticle systems induces...
Fabrication of Janus Micromotors Based on Black Titanium Dioxide for Medical Application
, M.Sc. Thesis Sharif University of Technology ; Madaah Hosseini, Hamid Reza (Supervisor) ; Tavakoli, Rohollah (Co-Supervisor)
Abstract
Enzyme-powered motors self-propel through the catalysis of biofuels, which makes them excellent candidates for biomedical applications. However, fundamental issues such as their movement in biological fluids and understanding the mechanism of propulsion are important aspects that must be considered before application in biomedicine. Building active systems based on biocompatible materials that use non-toxic fuels to power their vehicles have always been challenging. In this study, self-propelled micromotors consist of titanium dioxide black spheres asymmetrically coated with a thin layer of gold. Cysteine was used to bind urease enzyme due to thiol binding. By biocatalytically converting...
Complex self-propelled rings: A minimal model for cell motility
, Article Soft Matter ; Volume 13, Issue 35 , 2017 , Pages 5865-5876 ; 1744683X (ISSN) ; Dehghani Ghahnaviyeh, S ; Nejat Pishkenari, H ; Auth, T ; Gompper, G ; Sharif University of Technology
Royal Society of Chemistry
2017
Abstract
Collective behavior of active matter is observed for self-propelled particles, such as vibrated disks and active Brownian particles, as well as for cytoskeletal filaments in motile cells. Here, a system of quasi two-dimensional penetrable self-propelled rods inside rigid rings is used to construct a complex self-propelled particle. The rods interact sterically with each other and with a stationary or mobile ring via a separation-shifted Lennard-Jones potential. They either have a sliding attachment to the inside of the ring at one of their ends, or can move freely within the ring confinement. We study the inner structure and dynamics of the mobile self-propelled rings. We find that these...
Gaussian theory for spatially distributed self-propelled particles
, Article Physical Review E - Statistical, Nonlinear, and Soft Matter Physics ; Volume 94, Issue 6 , 2016 ; 15393755 (ISSN) ; Schimansky Geier, L ; Ejtehadi, M. R ; Sharif University of Technology
American Physical Society
2016
Abstract
Obtaining a reduced description with particle and momentum flux densities outgoing from the microscopic equations of motion of the particles requires approximations. The usual method, we refer to as truncation method, is to zero Fourier modes of the orientation distribution starting from a given number. Here we propose another method to derive continuum equations for interacting self-propelled particles. The derivation is based on a Gaussian approximation (GA) of the distribution of the direction of particles. First, by means of simulation of the microscopic model, we justify that the distribution of individual directions fits well to a wrapped Gaussian distribution. Second, we numerically...
Vortex with fourfold defect lines in a simple model of self-propelled particles
, Article Physical Review E - Statistical, Nonlinear, and Soft Matter Physics ; Volume 93, Issue 3 , 2016 ; 15393755 (ISSN) ; Ejtehadi, M. R ; Sharif University of Technology
American Physical Society
2016
Abstract
We study the formation of a vortex with fourfold symmetry in a minimal model of self-propelled particles, confined inside a squared box, using computer simulations and also theoretical analysis. In addition to the vortex pattern, we observe five other regimes in the system: a homogeneous gaseous phase, band structures, moving clumps, moving clusters, and vibrating rings. All six regimes emerge from controlling the strength of noise and from the contribution of repulsion and alignment interactions. We study the shape of the vortex and its symmetry in detail. The pattern shows exponential defect lines where incoming and outgoing flows of particles collide. We show that alignment and repulsion...