assist. prof. Žiga Kos
assist. prof. Žiga Kos
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Active colloids in complex fluids
Active colloids in complex fluids
Swimming at low Reynolds numbers requires non-reciprocal sequence of swimming strokes, which imposes strong constraints on possible propulsion mechanisms for microswimmers using shape deformations, beating flagella or cilia. On the contrary, in complex fluids, the rules of propulsion allow for much simpler swimming strokes—a radially oscillating air bubble can already propel in anisotropic fluids. In our research, we show several examples how externally actuated colloidal objects can swim in anisotropic fluids, formulating exact rules of propulsion and control mechanisms.
Selected publications:
Selected publications:
M. Rajabi, E. Caf, Q.X. Zhang, S.A. Crane, M. Ravnik, G. P. Alexander, Ž. Kos, K.J. Stebe, Scallop Theorem for Swimming in Anisotropic Fluids, arXiv:2509.22249 (2025). [Link] [PDF]
S. Kim, Ž. Kos, E. Um and J. Jeong, Symmetrically pulsating bubbles swim in an anisotropic fluid by nematodynamics, Nat. Commun. 15, 3 (2024). [Link] [PDF]
T. Yao, Ž. Kos, Q. X. Zhang, Y. Luo, E. B. Steager, M. Ravnik and K. J. Stebe, Topological defect-propelled swimming of nematic colloids, Science Advances 8, 20120255 (2022). [Link] [PDF]
T. Yao, Ž. Kos, Q. X. Zhang, Y. Luo, F. Serra, E. B. Steager, M. Ravnik and K. J. Stebe, Nematic Colloidal Micro‐Robots as Physically Intelligent Systems, Adv. Funct. Mater. 32, 2205546 (2022). [Link] [PDF]
Ž. Kos and M. Ravnik Elementary Flow Field Profiles of Micro-Swimmers in Weakly Anisotropic Nematic Fluids: Stokeslet, Stresslet, Rotlet and Source Flows, Fluids 3, 15 (2018). [Link]
Active nematics
Active nematics
Active matter consist of self-propelled active agents that consume energy from its environment. My research on active matter is linked to anisotropic active systems, such as bacterial collonies, tissues, and microtubule-kinesin mixtures. Using a mesoscopic approach and hybrid lattice Boltzmann numerical modelling, we show the non-equilibrium dynamics of 3D active nematics and focus specifically on the dynamics of topological defects that form spontaneously under sufficient activity or under geometrical constraints.
Selected publications:
Selected publications:
N. Kralj, M. Ravnik and Ž. Kos, Chirality, anisotropic viscosity and elastic anisotropy in three-dimensional active nematic turbulence, Commun. Phys. 7, 380 (2024). [Link] [PDF]
N. Kralj, M. Ravnik and Ž. Kos, Defect Line Coarsening and Refinement in Active Nematics, Phys. Rev. Lett. 130, 128101 (2023). [Link] [PDF]
J. Binysh, Ž. Kos, S. Copar, M. Ravnik, and G. P. Alexander, Three-Dimensional Active Defect Loops, Phys. Rev. Lett. 124, 088001 (2020) [Link] [PDF].
S. Čopar, J. Aplinc, Ž. Kos, S. Žumer and M. Ravnik Topology of three-dimensional active nematic turbulence confined to droplets, Phys. Rev. X, 9, 031051 (2019). [Link] [PDF].
P. Guillamat, Ž Kos, J. Hardoüin, J. I. Ignés-Mullol, M. Ravnik and F. Sagués Active nematic emulsions, Sci. Adv. 4, eaao1470 (2018). [Link] [PDF].
Non-equilibrium liquid crystals
Non-equilibrium liquid crystals
Nematic liquid crystals flowing inside microchannels show a rich variety of structures and are an ideal testing ground for non-equilibrium behaviour of nematic fluids. We explore analytically the structural dynamics of flowing nematic liquids due to flow-aligning interaction with the velocity field. By numerical modelling, we are also able to explore the nematic structure in junctions of microchannels and stabilize higher order defects. We also show, how optical fields can be used to periodically deform the director field, which could serve as a adjustable micropump with no moving parts.
Selected publications:
Selected publications:
M. Mur, Ž. Kos, M. Ravnik and I. Muševič, Continuous generation of topological defects in a passively driven nematic liquid crystal, Nat. Commun. 13, 387 (2022). [Link] [PDF]
S. Čopar, Ž. Kos, T. Emeršič, U.Tkalec, Microfluidic control over topological states in channel-confined nematic flows, Nat. Comm. 11 59 (2020) [Link] [PDF].
T. Emeršič, R. Zhang, Ž. Kos, S. Čopar, N. Osterman, J. J. de Pablo, and U. Tkalec, Sculpting stable structures in pure liquids, Sci. Adv. 5, 2 (2019) [Link] [PDF].
Ž. Kos and M. Ravnik, Field generated nematic microflows via backflow mechanism, Sci. Rep. 10, 1446 (2020). [Link] [PDF].
L. Giomi, Ž. Kos, M. Ravnik and A. Sengupta Cross-talk between topological defects in different fields revealed by nematic microfluidics , Proc. Natl. Acad. Sci., 114, 29 (2017). [Link] [PDF].
Complex soft matter systems
Complex soft matter systems
Most computers today are based on electronic digital circuts that manipulate binary sequences such as 0011001 with logic gates. Alternative computation methods are explored in soft matter, such as DNA-based computation, computation in cells, microfluidics and mechanical networks. Liquid crystals allow for production of large arrays of tunable structures and formation of complex topological states. We explore how such nematic liquid crystal states could be used as bits for computation and how logic operations could be applied to them.
Selected publications:
Selected publications:
C. Meng, J.S. Wu, Ž. Kos, J. Dunkel, C. Nisoli, I.I. Smalyukh, Emergent Dimer-Model Topological Order and Quasiparticle Excitations in Liquid Crystals: Combinatorial Vortex Lattices, Phys. Rev. X 15, 021084 (2025). [Link] [PDF]
S. Čopar and Ž. Kos, Many-defect solutions in planar nematics: interactions, spiral textures and boundary conditions, Soft Matter 20, 6894 (2024). [Link] [PDF]
V. P. Patil, Ž. Kos and J. Dunkel, Harmonic flow field representations of quantum bits and gates, Phys. Rev. Research 6, 043039 (2024). [Link] [PDF]
Ž. Kos and J. Dunkel, Nematic bits and universal logic gates, Sci. Adv. 8, eabp8371 (2022). [Link] [PDF]
In-silico modeling of biopharmaceutical processes
In-silico modeling of biopharmaceutical processes
The development of machine learning and neural network approaches has enabled major progress in protein science, including solving long-standing challenges such as the protein folding problem (recognized by the 2024 Nobel Prize in Chemistry). Computer modeling provides powerful tools to optimize biopharmaceutical processes and to gain insight into the underlying physical mechanisms. In my research, I collaborate with industrial partners to explore in-silico prediction of protein quality attributes, as well as modeling of chromatographic separation processes.
Selected publications:
Selected publications:
Prediction of aggregation rate by B. Knez et al / CC BY 4.0
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