40 research outputs found
Electrically driven convection in a thin annular film undergoing circular Couette flow
We investigate the linear stability of a thin, suspended, annular film of
conducting fluid with a voltage difference applied between its inner and outer
edges. For a sufficiently large voltage, such a film is unstable to
radially-driven electroconvection due to charges which develop on its free
surfaces. The film can also be subjected to a Couette shear by rotating its
inner edge. This combination is experimentally realized using films of smectic
A liquid crystals. In the absence of shear, the convective flow consists of a
stationary, azimuthally one-dimensional pattern of symmetric, counter-rotating
vortex pairs. When Couette flow is applied, an azimuthally traveling pattern
results. When viewed in a co-rotating frame, the traveling pattern consists of
pairs of asymmetric vortices. We calculate the neutral stability boundary for
arbitrary radius ratio and Reynolds number of the shear
flow, and obtain the critical control parameter and the critical azimuthal mode number . The
Couette flow suppresses the onset of electroconvection, so that . The calculated suppression is
compared with experiments performed at and .Comment: 17 pages, 2 column with 9 included eps figures. See also
http://mobydick.physics.utoronto.c
Bifurcations in annular electroconvection with an imposed shear
We report an experimental study of the primary bifurcation in
electrically-driven convection in a freely suspended film. A weakly conducting,
submicron thick smectic liquid crystal film was supported by concentric
circular electrodes. It electroconvected when a sufficiently large voltage
was applied between its inner and outer edges. The film could sustain rapid
flows and yet remain strictly two-dimensional. By rotation of the inner
electrode, a circular Couette shear could be independently imposed. The control
parameters were a dimensionless number , analogous to the Rayleigh
number, which is and the Reynolds number of the
azimuthal shear flow. The geometrical and material properties of the film were
characterized by the radius ratio , and a Prandtl-like number . Using measurements of current-voltage characteristics of a large number of
films, we examined the onset of electroconvection over a broad range of
, and . We compared this data quantitatively to
the results of linear stability theory. This could be done with essentially no
adjustable parameters. The current-voltage data above onset were then used to
infer the amplitude of electroconvection in the weakly nonlinear regime by
fitting them to a steady-state amplitude equation of the Landau form. We show
how the primary bifurcation can be tuned between supercritical and subcritical
by changing and .Comment: 17 pages, 12 figures. Submitted to Phys. Rev. E. Minor changes after
refereeing. See also http://mobydick.physics.utoronto.c
Mapping single-cell data to reference atlases by transfer learning
Large single-cell atlases are now routinely generated to serve as references for analysis of smaller-scale studies. Yet learning from reference data is complicated by batch effects between datasets, limited availability of computational resources and sharing restrictions on raw data. Here we introduce a deep learning strategy for mapping query datasets on top of a reference called single-cell architectural surgery (scArches). scArches uses transfer learning and parameter optimization to enable efficient, decentralized, iterative reference building and contextualization of new datasets with existing references without sharing raw data. Using examples from mouse brain, pancreas, immune and whole-organism atlases, we show that scArches preserves biological state information while removing batch effects, despite using four orders of magnitude fewer parameters than de novo integration. scArches generalizes to multimodal reference mapping, allowing imputation of missing modalities. Finally, scArches retains coronavirus disease 2019 (COVID-19) disease variation when mapping to a healthy reference, enabling the discovery of disease-specific cell states. scArches will facilitate collaborative projects by enabling iterative construction, updating, sharing and efficient use of reference atlases