Search Results (All Fields:"size", Date:" [2019\-01\-01T00\:00\:00Z TO 2019\-12\-31T00\:00\:00Z] ", isMemberOf:"bibliuned:Setopenaire") - e-spacio
http://e-spacio.uned.es/fez/
Universidad Nacional de Educación a DistanciaspFez 2.1 RC3http://blogs.law.harvard.edu/tech/rssNanoscale hydrodynamics near solids
http://e-spacio.uned.es/fez/view/tesisuned:ED-Pg-Ciencias-Dduque
This dissertation studies the behaviour of a fluid in contact with a solid in the nanoscale. Theoretical formulations of non-local continuum and discrete hydrodynamics are presented in which the interaction of the solid with the fluid appears explicitly in terms of extended forces on the fluid, confined to the vicinity of the solid object. The discrete theory is validated through MD simulations, where we encounter the plateau problem in the determination of the transport coefficients. We offer a method that solves the problem and allows us to evaluate the transport coefficients unambigously. In the course of the MD investigation we find that the Markovian assumption implicit in the theoretical derivations is not satisfied near the wall when the hydrodynamics is resolved at molecular scales. However, for sufficiently large bins in which the discrete hydrodynamic variables are defined the behaviour is fully Markovian. The final outcome of the present dissertation is the derivation of the slip boundary condition from the microscopically formulated discrete hydrodynamic theory. The slip length and the position of the wall are defined through Green-Kubo formulas and seen to coincide with the original proposal of Bocquet and Barrat. We test the validity of the slip boundary condition thus obtained in a particularly challenging flow, an initial plug flow that is discontinuous near the wall at initial stages of the flow. We observe that the slip boundary condition is violated at the initial stages of the flow and we explain the reasons for this failure. More specifically,we derive, using the Theory of Coarse-Graining (ToCG), the equations of motion of a fluid in contact with a solid sphere of large dimensions compared to molecular scales. We use the Kawasaki-Gunton projection operator technique which leads to a set of nonlinear equations for the relevant variables of the system. We assume the Markovian approximation in order to obtain a set of memoryless equations. We address the well-known plateau problem that appears in the expressions of the transport coefficients present in the equations of motion of the fluid in contact with a solid. We solve this problem and we obtain an alternative expression for the transport coefficients without the plateau problem. In order to validate the theory and measure the transport coefficients with Molecular Dynamics (MD) simulations, we derive the discrete equations of motion for a set of discrete hydrodynamic variables. These variables are defined in terms of finite element basis functions based on regular bins constructed by dividing the domain of the fluid with equiespaced parallel planes. The discrete set of hydrodynamic equations are shown to be identical to a Petrov-Galerkin discretization of the continuum equations. The only assumption made in order to derive the equations of motion of the fluid is the Markovian hypothesis. In order to validate it, we use Mori theory which let us obtain the time evolution of the correlation of the relevant variables of the system. The equations are linear, implying an exponential behaviour of the correlation. We take deviations from exponential decay as an indication of the violation of the Markovian hypothesis. As a first step to address the problem of Markovianity we study the simpler case of a fluid in periodic boundary conditions (PBC) by performing MD simulations. We monitor the relevant variales of the system and compute their correlations. We realize that is necessary to move to the reciprocal space (i.e. the Fourier space for unconfined fluids) in order to validate the Markovian hypothesis. Only for an exponential decay of the modes of the matrix of correlations we can expect the hypothesis to be valid. Once the methodology is well established we address the more complicated case of a confined fluid between two solid slabs. We show that if the size of the bin is of the order of the molecular length the Markovian hypothesis fails for modes near the walls. Nevertheless, for large bins the hypothesis is satisfied but the nonlocal effect of layering is lost. Finally, we derive the slip boundary condition from the theory proposed, by considering mechanical balance within a slab of fluid near the wall, and assuming that the flow field inside this slab is linear. This allows to infer the slip length and the position of the wall where the boundary condition is to be satisfied in microscopic terms, through Green-Kubo expressions. The microscopic expression obtained coincides with the original proposal by Bocquet and Barrat. We demonstrate that the friction coefficient is an intrinsic property of the surface of the solid as it does not depend on the width of the channel.2019-08-30T21:34:01Z
Duque Zumajo, Diego
Video-game training effects on attention and memory in young and older adults. Behavioral results
http://e-spacio.uned.es/fez/view/tesisuned:ED-Pg-PsiSal-Eruiz
This Thesis is organized in 7 chapters: the theoretical framework, the cognitive changes associated with normal and pathological aging and the contribution of video games programs to prevent the cognitive decline at different age stages, two publications of the interventions with video games in young and older adults, and the main conclusions of the current work. The number of publications related to the effects of video-game training has increased substantially in recent decades, from 15 papers per year during the 90s, to 350 papers in 2015 (Palaus, Marron, Viejo-Sobera, & Redolar-Ripoll, 2017). Video games have many advantages in terms of availability, engagement and cost (Joddrell & Astell, 2016; Burgers, Eden, Van Engelenburg, & Buningh, 2015) but their neurocognitive benefits are still subject to debate. Some authors found positive results derived from video game training (Cain, Landau, & Shimamura, 2012; Shawn Green & Bavelier, 2003; Mondéjar, Hervás, Johnson, Gutierrez, & Latorre, 2016), while others could not replicate them or find any benefit (Irons, Remington, & McLean, 2011; Kable et al. 2017; Murphy & Spencer, 2009). Due to this lack of consensus in the literature about the neurocognitive benefits of video-game training, many studies have pointed out the importance of using an appropriate methodological approach for these kinds of interventions. “The gold standard design” consists in a double blinded study with randomization of participants and an appropriate active control group that matches the experimental group, where placebo effects depending on motivation, engagement and expectations are controlled, and which measures the transfer of learning from the intervention to the neurocognitive functions under study (Boot et al., 2011; Boot & Simons, 2012; Boot et al., 2013; Simons et al., 2016; Shawn Green, Strobach, & Schubert, 2014; Dale & Shawn Green, 2017; García- Betances, Cabrera-Umpiérrez, & Arredondo, 2017; Schubert, Strobach, & Karbach, 2014; Seitz, 2018). Trying to follow these guidelines, we conducted two randomized interventions to analyze the effects of brain-game training on memory and attentional functions in young and older adults. A recent meta-analysis of action video game training studies found that healthy young and older adults benefited from training in overall and specific cognitive domains, but that young adults benefited more than older adults (Wang et al., 2017). It has been studied that brain neuroplasticity continues throughout the lifespan, but how the changes are regulated depends on age (Kolb & Muhammad, 2014; Merzenich, Van Vleet, & Nahum, 2014). Neurocognitive decline associated with normal aging mainly affects memory (working memory, episodic memory, information retrieval and consolidation), executive functions (decision making, problem solving and mental control), attentional networks (selective and divided attention and visuospatial abilities) and reaction times. Thus, it is a priority to investigate the effects of cognitive stimulation programs and optimize them. Among these programs, video games offer many advantages because they are enjoyable, adaptive and engaging for aging people (Chesham, Wyss, Müri, Mosimann, & Nef, 2017) Moreover, neurocognitive deficits associated with normal aging usually occur in pathological aging, with differences in degree, intensity, frequency and temporal distribution (de Flores, La Joie, & Chételat, 2015; Hullinger & Puglielli, 2017). It is therefore essential to study normal and pathological aging, the mechanisms that these processes share, and the stimulation programs that could help to avoid or attenuate them. We firstly conducted an intervention with healthy older adults to measure the effects of non-action adaptive mental games on the cognition of older and young participants, specifically working memory, selective attention, distraction and response inhibition. The results showed that brain games and other kinds of video games such as simulation games could positively benefit the attention and working memory of older adults, but the effects are moderate in terms of near and far transfer of learning. However, older adults showed no gains in inhibitory control after training with non-action video games. Then, we conducted a second intervention with adults aging between 18 and 35 years old. Analysis showed that young adults can transfer learning from non-action adaptive brain games to attention and visuo-spatial working memory with moderate effect sizes, and the positive results could also be obtained with other non-action video games such as simulation games. Nonetheless, playing non-action video games did not improve inhibition responses in young adults either. However, as we did not have a non- intervention control group, we cannot conclude that adaptive non-active video games had an effect. Future studies should include both an active control group and a no-contact group.2019-11-07T20:00:43Z
Ruíz Márquez, Eloísa