Colloidal Particles at Chiral Liquid Crystal Interfaces

Colloidal particles trapped at an interface between two fluids can form a wide range of different structures. Replacing one of the fluid with a liquid crystal increases the complexity of interactions and results in a greater range of possible structures. New behaviour emerges when colloidal particles interact with defects in the liquid crystal phases. Here we discuss the templating of colloids at a cholesteric isotropic interface.Comment: 7 pages, 5 figure

Hydrodynamic oscillations and variable swimming speed in squirmers close to repulsive walls

We present a lattice Boltzmann study of the hydrodynamics of a fully resolved squirmer, radius R, confined in a slab of fluid between two no-slip walls. We show that the coupling between hydrodynamics and short-range repulsive interactions between the swimmer and the surface can lead to hydrodynamic trapping of both pushers and pullers at the wall, and to hydrodynamic oscillations in the case of a pusher. We further show that a pusher moves significantly faster when close to a surface than in the bulk, whereas a puller undergoes a transition between fast motion and a dynamical standstill according to the range of the repulsive interaction. Our results critically require near-field hydrodynamics; they further suggest that it should be possible to control density and speed of squirmers at a surface by tuning the range of steric and electrostatic swimmer-wall interactions.Comment: 5 + 8 pages, 4 + 4 Figure

Towards improved simulations of self-organising molecular materials

Computer simulations can be used in parallel with experimental techniques to gain valuable insights into physical systems, test theoretical models or predict new be- haviour of molecular materials. Long time and large length scales, in combination with problems of phase space sampling, present a grand challenge for simulations of self-organising molecular materials. In the work presented in this thesis, the aim has been to develop and apply new or recent simulation models and methods to address these issues, with the aim of producing improved simulations of molecular materials. A new anisotropic model for simulating mesogenic systems has been developed, based on a soft core spherocylinder potential. This model is tested for single site systems and a multipedal liquid crystalline molecule, using conventional molecular dynamics simulations. It is used also to map out an approximate phase diagram for a main chain liquid crystalline polymer as a function of the volume fraction of the mesogenic unit; and to study the effects of a chiral medium on flexible achiral dopant molecules. Results here, show a preferential selection of conformations of similar chirality to the solvent. Later in the thesis, this new soft core spherocylinder model, is combined with a recently developed simulation methdology, Statistical Temperature Molecular Dynamics, to study the isotropic-nematic phase transition of a single site mesogen and the isotropic-lamellar phase transition of a model rod- coil diblock copolymer, using a single simulation to span the temperature window corresponding to the phase transition. Additional simulations combine a mesoscopic simulation method, Stochastic Ro- tational Dynamics, with a coarse grained surfactant model. This allows a computa- tionally efficient solvent description while maintaining correct hydrodynamics. Re- sults presented here include the formation of a bilayer, via spontaneous self-assembly of surfactant molecules, and information on the pathways of micelle formation. In the final result chapter of this thesis, Hamiltonian replica exchange simulations are performed employing soft-core replicas for a Gay-Berne system. The simulation results show an order of magnitude increase in equilibration speed of the ordered phase when compared to conventional simulations of a Gay-Berne fluid

Collective Flows Drive Cavitation in Spinner Monolayers

Hydrodynamic interactions can give rise to a collective motion of rotating particles. This, in turn, can lead to coherent fluid flows. Using large scale hydrodynamic simulations, we study the coupling between these two in spinner monolayers at weakly inertial regime. We observe an instability, where the initially uniform particle layer separates into particle void and particle rich areas. The particle void region corresponds to a fluid vortex, and it is driven by a surrounding spinner edge current. We show that the instability originates from a hydrodynamic lift force between the particle and fluid flows. The cavitation can be tuned by the strength of the collective flows. It is suppressed when the spinners are confined by a no-slip surface, and multiple cavity and oscillating cavity states are observed when the particle concentration is reduced

Perusopetuksen tuen tarjonnan muutokset 1970–2016 – erityisopetuksesta oppimisen ja koulunkäynnin tukeen

Peer reviewe

Bulk rheology and microrheology of active fluids

We simulate macroscopic shear experiments in active nematics and compare them with microrheology simulations where a spherical probe particle is dragged through an active fluid. In both cases we define an effective viscosity: in the case of bulk shear simulations this is the ratio between shear stress and shear rate, whereas in the microrheology case it involves the ratio between the friction coefficient and the particle size. We show that this effective viscosity, rather than being solely a property of the active fluid, is affected by the way chosen to measure it, and strongly depends on details such as the anchoring conditions at the probe surface and on both the system size and the size of the probe particle.Comment: 12 pages, 10 figure

Using tablet devices to control complex home appliances

Internet of things has made connected devices and appliances widely available and tablet devices are common household items. This study focuses on technical user interface design challenges and requirements for user interface design of controlling complex home appliances with tablet devices. There is a literature review about available controlling technologies and usability heuristics related to tablet and mobile devices. An Android test application was created and tested with four test users to find out how well those heuristics work and are covered. That application was tested against the regular user interface of a dishwasher and task completion times and errors were noted down. Test users were asked to answer a questionnaire regarding the heuristics and how well the implementation performed. Tablet devices should be evaluated using regular usability heuristics, but besides them they require mobile specific heuristics, such as easy of input, screen readability and glancability, physical interaction and ergonomics and privacy and social convention taken into account. The results showed that a tablet user interface was able to outperform its regular counterpart in task completion times and in number of errors. The implementation also covered those heuristics in a more comprehensive way. But among test persons the most benefit was with users who were familiar with tablets and not with dishwashers. A test user who wasn t familiar with tablets but was with dishwashers performed tasks faster and with fewer errors with regular user interface. In conclusion a tablet user interface enabled users who were familiar with tablets to perform tasks faster and with less errors. Those users were also more satisfied with a tablet user interface than a regular one. On the other hand a test user with little experience of tablets and familiarity with dishwashers was able to perform tasks faster an with less errors with the regular user interface. A tablet user interface was able to offer extra benefits and efficiency to users, but regular user interface should be also available to satisfy users who are not familiar with mobile devices