418 research outputs found
Modelling evaporation and phase behaviour of particle suspensions
We present two statistical mechanics based methods for simulating the evaporation of droplets of nanoparticle suspensions from upon a heterogeneous surface. These are based on a generalised lattice-gas model. Properties such as wettability and the dynamic contact angle, are determined by the attraction strength parameters between particles and from the dynamic mobility coefficients. Both models incorporate the effects of surface roughness and slip at the surface. The two approaches used are Monte Carlo (MC) computer simulations and Dynamical Density Functional Theory (DDFT). We calculate the bulk fluid phase behaviour including the influence of the suspended nanoparticles, comparing results from the two approaches. We also calculate thermodynamic quantities such as the surface tensions. Our results show that the presence of steps in the surface can be crucial in controlling dewetting from heterogeneous surfaces. We also observe that coffee ring stains can be formed via the coupling of evaporation to phase separation and that the advective hydrodynamics within the droplets in not required for ring stains to form
Recommended from our members
Hierarchical dynamics of individual RNA helix base pair formation and disruption
This thesis explores the RNA folding problem using single-molecule field effect transistors (smFETs) to measure the lifetimes of individual RNA base-pairing rearrangements. In the course of this research, considerable computational, chemical, and engineering contributions were developed so that the single-molecule measurements could be conducted and quantified. These advancements have allowed, on the basis of the smFET data collected herein, the quantification of a kinetic model for RNA stem-loop structures which has been generalized to quantitatively explore the phenomenological observation that an RNA found in the bacillus subtilis strain acts as a metabolite-sensing switch, allowing RNA polymerase to transcribe the messenger RNA when the metabolite is present and preventing transcription when the metabolite is absent. Together, the data presented quantify a simple model for the base pairing rearrangements that underlie RNA folding
Recommended from our members
On exciton-vibration and exciton-photon interactions in organic semiconductors
Organic semiconductors are materials that are promising for novel optoelectronic applications, such as more efficient solar cells and LEDs. The optoelectronic response of these materials is dominated by bound electron-hole pairs called excitons, which are often strongly affected by hundreds of possible molecular vibrations. Although quantum theory contains all the ingredients to describe these complex phenomena, in practice it is only possible to solve the corresponding equations in small systems with few vibrations. As a result, it has been common to assume weak exciton-vibration interactions and to employ perturbative approaches. Similarly, exciton-photon interactions have almost universally been treated in the so-called weak coupling regime. However, in recent years it has become increasingly clear that these approximations can break down in organic semiconductors, placing an important roadblock towards the novel energy-harvesting technologies that could be based on these materials.
In this thesis we address this issue by developing methods to treat exciton-photon and exciton-vibration interactions, without relying on any approximation regarding their magnitude. We propose a first principles description of hybrid exciton-light (polariton) states that result from strong exciton-photon interactions. We discuss a method to treat strong exciton-vibration interactions, showing that the spatial extent of exciton states controls their magnitude. Subsequently, we present a beyond Born-Oppenheimer method based on tensor networks to study real-time exciton dynamics. By using these methods, we show how selective excitation of vibrational modes can enhance charge transfer. Moreover, through rigorous comparison to experiments, we highlight that tensor network methods are highly accurate, and we generate a `movie' of the photophysical process of singlet fission, which occurs during early light-harvesting by organic molecules and has the potential to increase solar cell efficiencies. Finally, we construct a singlet fission model including the effects of excess energy, vibrations and the solvent of molecules concurrently, demonstrating that the fission mechanism can be qualitatively changed in a controlled manner, allowing for its acceleration by an order of magnitude.Winton Programme for the Physics of Sustainabilit
Non-Hermitian Topological Magnonics
Dissipation in mechanics, optics, acoustics, and electronic circuits is
nowadays recognized to be not always detrimental but can be exploited to
achieve non-Hermitian topological phases or properties with functionalities for
potential device applications. As elementary excitations of ordered magnetic
moments that exist in various magnetic materials, magnons are the information
carriers in magnonic devices with low-energy consumption for reprogrammable
logic, non-reciprocal communication, and non-volatile memory functionalities.
Non-Hermitian topological magnonics deals with the engineering of dissipation
and/or gain for non-Hermitian topological phases or properties in magnets that
are not achievable in the conventional Hermitian scenario, with associated
functionalities cross-fertilized with their electronic, acoustic, optic, and
mechanic counterparts, such as giant enhancement of magnonic frequency combs,
magnon amplification, (quantum) sensing of the magnetic field with
unprecedented sensitivity, magnon accumulation, and perfect absorption of
microwaves. In this review article, we address the unified approach in
constructing magnonic non-Hermitian Hamiltonian, introduce the basic
non-Hermitian topological physics, and provide a comprehensive overview of the
recent theoretical and experimental progress towards achieving distinct
non-Hermitian topological phases or properties in magnonic devices, including
exceptional points, exceptional nodal phases, non-Hermitian magnonic SSH model,
and non-Hermitian skin effect. We emphasize the non-Hermitian Hamiltonian
approach based on the Lindbladian or self-energy of the magnonic subsystem but
address the physics beyond it as well, such as the crucial quantum jump effect
in the quantum regime and non-Markovian dynamics. We provide a perspective for
future opportunities and challenges before concluding this article.Comment: 101 pages, 35 figure
Dynamics of ion Coulomb crystals
The field of quantum simulations has achieved a remarkable success through the
development of highly controllable and accessible quantum platforms, which pro-
vide insights into the microscopic properties of complex large-scale systems that
are otherwise difficult to analyze. Many of the platforms utilized in this pursuit are
derived from the field of atomic, molecular, and optical physics. One particularly
popular candidate is provided by trapped ions, whose vibrational and electronic
degrees of freedom can be effectively combined through laser pulses to engineer
desired model Hamiltonians or quantum circuits. Trapped ions constitute as well
the basis for modern atomic clocks, the most precise frequency standards currently
available. They find further applications in metrology, geodesy, and fundamental
physics experiments.
In this Thesis, we investigate the dynamics of vibrational modes in trapped
ion crystals, utilizing them as a versatile platform to explore various many-body
phenomena.
We first focus on the expansion dynamics of local excitations and on heat
transport within ion crystals hosting structural defects that undergo a sliding-
to-pinned transition. We observe a significant reduction in conductivity when
the crystal symmetry is spontaneously broken during the transition, and show
that resonances between crystal eigenmodes lead to distinct softening signatures
associated with energy localization. We then delve into the effects of thermal and
quantum fluctuations on the vibrational modes of ion crystals near two distinct
structural transitions. We observe the emergence of a prolonged symmetric phase
stabilized by thermal and quantum fluctuations, and develop effective theories that
reduce the degrees of freedom to the modes that drive the transitions.
Finally, we discuss how to engineer spin-orbit coupling and on-site interaction
energies for vibrational quantum excitations using two different external driving
schemes. While the simulation of spin models with ions typically involves the use
of two electronic states, we propose interpreting the two local oscillation modes
in an ion crystal as a pseudospin. We show how using Floquet engineering ideas
allows for spin flips in Coulomb-induced vibron hopping, resulting in a non-trivial
coupling between spatial motion and spin evolution, that results in a markedly non-Abelian dynamics. Subsequently, we explore the simulation of Hubbard models in
trapped ions by coupling the vibrational Fock states to an internal level system.
Our findings include the observation of bound states in the strong interaction limit
of the resulting Jaynes-Cummings-Hubbard model.
By investigating these topics, we aim to contribute to the understanding of
vibrational dynamics in trapped ion crystals, and shed light on their potential for
simulating condensed matter systems, offering insights into phenomena that are
otherwise challenging to explore.DFG/Sonderforschungsbereich 1227 DQ-mat/274200144/E
- …