| Izvleček: | <p>Active materials take energy from their surroundings and use it to do mechanical work. The theories of active matter can describe a wide variety of living systems, such as bacterial colonies, actin-myosin networks and cell tissues. In this thesis, we investigate the collective behaviour of confluent, two-dimensional cell monolayers on a substrate, which is also one of the most common experimental systems to study collective cell motility. The cells are modelled using a computational phase field approach to study how polar and dipolar forces, the active, out-of-equilibrium force components, contribute to the cell motions.</p>
<p>We study the polar and dipolar contributions to collective cell motility using the phase-field model, which allows simulations of deformable cells. We study the interplay between the polar and dipolar forces in a confluent cell sheet and explore the collective behaviour emerging from different polarisation mechanisms. We find that either polar or dipolar forcing can drive the jammed-to-liquid transition. The mechanical properties, such as the cell deformation and speed, are independent of the polarisation mechanisms. However, a flocking state where most cells move in the same direction appears when the polar force of a cell aligns with its velocity as an exception.</p>
<p>Then we focus on studying the active inter-cellular force in a cell monolayer. We find that contractile inter-cellular forces elongate a cell and can give rise to a nematic state and active turbulence. Extensile inter-cellular forces cause frustration, where cells elongate perpendicular to their neighbours. Moreover, we show that anisotropic fluctuation in the inter-cellular forces can lead to the transition from contractile to extensile behaviour.</p>
<p>Finally, we investigate the interplay between the viscoelasticity of tissue and the inter-cellular forces for cells confined in a channel. We find that different collective behaviour emerges as viscoelasticity of the tissue is modulated by the cell-cell adhesion, repulsion. The transition between the jammed, nematic state,
oscillation and spontaneous shear flow occurs as the cell-cell adhesion increases. The cells jam more easily with higher cell-cell repulsion, and cells of a higher aspect ratio favour steady shear. </p>
<p>The approaches developed in this thesis can be extended and applied to investigate other
biological processes in cell mechanics, such as tissue expansion and the phase ordering of different cell types. </p>
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