386 research outputs found
On the anomalous dynamics of capillary rise in porous media
The anomalous dynamics of capillary rise in a porous medium discovered
experimentally more than a decade ago (Delker et al., Phys. Rev. Lett. 76
(1996) 2902) is described. The developed theory is based on considering the
principal modes of motion of the menisci that collectively form the wetting
front on the Darcy scale. These modes, which include (i) dynamic wetting mode,
(ii) threshold mode and (iii) interface de-pinning process, are incorporated
into the boundary conditions for the bulk equations formulated in the regular
framework of continuum mechanics of porous media, thus allowing one to consider
a general case of three-dimensional flows. The developed theory makes it
possible to describe all regimes observed in the experiment, with the time
spanning more than four orders of magnitude, and highlights the dominant
physical mechanisms at different stages of the process
Topological defect lasers
We demonstrate topological defect lasers in a GaAs membrane with embedded
InAs quantum dots. By introducing a disclination to a square-lattice of
elliptical air holes, we obtain spatially confined optical resonances with high
quality factor. Such resonances support powerflow vortices, and lase upon
optical excitation of quantum dots, embedded in the structure. The spatially
inhomogeneous variation of the unit cell orientation adds another dimension to
the control of a lasing mode, enabling the manipulation of its field pattern
and energy flow landscape
Investigation of the Dimensional Variation of Microstructures Through the ÎĽMIM Process
The mass production of components with dimensions in the micron and sub-micron range is anticipated to be one of the leading technology areas for the present century and to be of high market potential. Micro metal injection molding (ÎĽMIM) has the potential to be an important contributor to this industry as it can produce precise metallic microstructures in large quantities at a relatively low production cost. The ÎĽMIM process is a miniaturization of metal injection molding (MIM) methods. The process comprises of four main steps: mixing, injection molding, debinding and sintering. A metallic powder is mixed with a binder system to form the feedstock. The feedstock is then
injection molded into the required shape and the binder removed via thermal or other means. The final microstructures are obtained by sintering the remaining powder in a controlled
environment. In this work, the dimensional variation of the microstructures, in particular the warpage, roughness and volume variation, at each stage of the ÎĽMIM process was quantified and compared. The results of a preliminary study of the sensitivity of warpage of the microstructures to the
packing pressure are also reported.Singapore-MIT Alliance (SMA
The role of input noise in transcriptional regulation
Even under constant external conditions, the expression levels of genes
fluctuate. Much emphasis has been placed on the components of this noise that
are due to randomness in transcription and translation; here we analyze the
role of noise associated with the inputs to transcriptional regulation, the
random arrival and binding of transcription factors to their target sites along
the genome. This noise sets a fundamental physical limit to the reliability of
genetic control, and has clear signatures, but we show that these are easily
obscured by experimental limitations and even by conventional methods for
plotting the variance vs. mean expression level. We argue that simple, global
models of noise dominated by transcription and translation are inconsistent
with the embedding of gene expression in a network of regulatory interactions.
Analysis of recent experiments on transcriptional control in the early
Drosophila embryo shows that these results are quantitatively consistent with
the predicted signatures of input noise, and we discuss the experiments needed
to test the importance of input noise more generally.Comment: 11 pages, 5 figures minor correction
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