39 research outputs found
Phase coexistence in a monolayer of active particles induced by Marangoni flows
Thermally or chemically active colloids generate thermodynamic gradients in
the solution in which they are immersed and thereby induce hydrodynamic flows
that affect their dynamical evolution. Here we study a mean-field model for the
many-body dynamics of a monolayer of active particles located at a fluid-fluid
interface. In this case, the activity of the particles creates long-ranged
Marangoni flows due to the response of the interface, which compete with the
direct interaction between the particles. For the most interesting case of a
soft repulsion that models the electrostatic or magnetic interparticle
forces, we show that an "onion-like" density distribution will develop within
the monolayer. For a sufficiently large average density, two-dimensional phase
transitions (freezing from liquid to hexatic, and melting from solid to
hexatic) should be observable in a radially stratified structure. Furthermore,
the analysis allows us to conclude that, while the activity may be too weak to
allow direct detection of such induced Marangoni flows, it is relevant as a
collective effect in the emergence of the experimentally observable spatial
structure of phase coexistences noted above. Finally, the relevance of these
results for potential experimental realizations is critically discussed.Comment: 11 page
Effective interaction between active colloids and fluid interfaces induced by Marangoni flows
We show theoretically that near a fluid-fluid interface a single active
colloidal particle generating, e.g., chemicals or a temperature gradient
experiences an effective force of hydrodynamic origin. This force is due to the
fluid flow driven by Marangoni stresses induced by the activity of the
particle; it decays very slowly with the distance from the interface, and can
be attractive or repulsive depending on how the activity modifies the surface
tension. We show that, for typical systems, this interaction can dominate the
dynamics of the particle as compared to Brownian motion, dispersion forces, or
self-phoretic effects. In the attractive case, the interaction promotes the
self-assembly of particles into a crystal-like monolayer at the interface.Comment: The manuscript proper and the supplementary information have been
merged consecutively into a single PDF fil
Effective Interaction between Active Colloids and Fluid Interfaces Induced by Marangoni Flows
We show theoretically that near a fluid-fluid interface a single active colloidal particle generating, e.g., chemicals or a temperature gradient experiences an effective force of hydrodynamic origin. This force is due to the fluid flow driven by Marangoni stresses induced by the activity of the particle; it decays very slowly with the distance from the interface, and can be attractive or repulsive depending on how the activity modifies the surface tension. We show that, for typical systems, this interaction can dominate the dynamics of the particle as compared to Brownian motion, dispersion forces, or self-phoretic effects. In the attractive case, the interaction promotes the self-assembly of particles into a crystal-like monolayer at the interface.COST Action MP1106European Cooperation in Science and Technology MP110
Self-Motility of an Active Particle Induced by Correlations in the Surrounding Solution
Current models of phoretic transport rely on molecular forces creating a “diffuse” particle-fluid interface. We investigate theoretically an alternative mechanism, in which a diffuse interface emerges solely due to a nonvanishing correlation length of the surrounding solution. This mechanism can drive self-motility of a chemically active particle. Numerical estimates indicate that the velocity can reach micrometers per second. The predicted phenomenology includes a bilinear dependence of the velocity on the activity and a possible double velocity reversal upon varying the correlation length.Spanish Government through Grant No. FIS2017-87117-P (partially financed by FEDER funds
Floor- or ceiling-sliding for chemically active, gyrotactic, sedimenting Janus particles
Surface bound catalytic chemical reactions self-propel chemically active
Janus particles. In the vicinity of boundaries, these particles exhibit rich
behavior, such as the occurrence of wall-bound steady states of "sliding". Most
active particles tend to sediment as they are density mismatched with the
solution. Moreover Janus spheres, which consist of an inert core material
decorated with a cap-like, thin layer of a catalyst, are gyrotactic
("bottom-heavy"). Occurrence of sliding states near the horizontal walls
depends on the interplay between the active motion and the gravity-driven
sedimentation and alignment. It is thus important to understand and quantify
the influence of these gravity-induced effects on the behavior of model
chemically active particles moving near walls. For model gyrotactic,
self-phoretic Janus particles, here we study theoretically the occurrence of
sliding states at horizontal planar walls that are either below ("floor") or
above ("ceiling") the particle. We construct "state diagrams" characterizing
the occurrence of such states as a function of the sedimentation velocity and
of the gyrotactic response of the particle, as well as of the phoretic mobility
of the particle. We show that in certain cases sliding states may emerge
simultaneously at both the ceiling and the floor, while the larger part of the
experimentally relevant parameter space corresponds to particles that would
exhibit sliding states only either at the floor or at the ceiling or there are
no sliding states at all. These predictions are critically compared with the
results of previous experimental studies and our experiments conducted on
Pt-coated polystyrene and silica-core particles suspended in aqueous hydrogen
peroxide solutions.Comment: Total number of pages: 33, Number of figures: 18. The video files, as
mentioned in the supplementary material will be provided by the corresponding
author upon reques
Exponential dichotomies of evolution operators in Banach spaces
This paper considers three dichotomy concepts (exponential dichotomy, uniform
exponential dichotomy and strong exponential dichotomy) in the general context
of non-invertible evolution operators in Banach spaces. Connections between
these concepts are illustrated. Using the notion of Green function, we give
necessary conditions and sufficient ones for strong exponential dichotomy. Some
illustrative examples are presented to prove that the converse of some
implication type theorems are not valid
Predicting disease risks from highly imbalanced data using random forest
<p>Abstract</p> <p>Background</p> <p>We present a method utilizing Healthcare Cost and Utilization Project (HCUP) dataset for predicting disease risk of individuals based on their medical diagnosis history. The presented methodology may be incorporated in a variety of applications such as risk management, tailored health communication and decision support systems in healthcare.</p> <p>Methods</p> <p>We employed the National Inpatient Sample (NIS) data, which is publicly available through Healthcare Cost and Utilization Project (HCUP), to train random forest classifiers for disease prediction. Since the HCUP data is highly imbalanced, we employed an ensemble learning approach based on repeated random sub-sampling. This technique divides the training data into multiple sub-samples, while ensuring that each sub-sample is fully balanced. We compared the performance of support vector machine (SVM), bagging, boosting and RF to predict the risk of eight chronic diseases.</p> <p>Results</p> <p>We predicted eight disease categories. Overall, the RF ensemble learning method outperformed SVM, bagging and boosting in terms of the area under the receiver operating characteristic (ROC) curve (AUC). In addition, RF has the advantage of computing the importance of each variable in the classification process.</p> <p>Conclusions</p> <p>In combining repeated random sub-sampling with RF, we were able to overcome the class imbalance problem and achieve promising results. Using the national HCUP data set, we predicted eight disease categories with an average AUC of 88.79%.</p