Latest Research Topics
Error Correction Techniques for Phase-Change Memories
Abstract
To mitigate the DRAM scalability challenges alternative memory technologies including several emerging non-volatile memories are being explored as possible DRAM replacements. Phase Change Memory (PCM) is one such emerging technology which appears to be the forerunner being closest to mass production. One of the main roadblocks for wider adoption of PCM is the limited write endurance, which leads to wear-out related permanent failures. Furthermore, PCM devices using multi-level cells so as to increase the density suffer from resistance drift induced transient faults. In the first part of this talk, I will present, SAFER, a hardware-efficient multi-bit stuck-at fault error recovery scheme which exploits the key attribute that a failed cell with a stuck-at value is still readable, making it possible to continue to use the failed cell to store data; thereby reducing the hardware overhead for error recovery. In the second part of the talk, I will present techniques for resistance drift management in PCM with multi-level cells. These architecture-level techniques combine lightweight reads for approximate error detection, and support synergistic scrub algorithms to improve device lifetime in the presence of resistance drift.
To Read more: http://www.csa.iisc.ernet.in/sem-evts/seminars.php#2012-02-03-1600-03
Functional encryption systems from hard lattice problems
Abstract
Data security challenges faced in the modern world demand functionality from encryption systems that traditional public key cryptography falls far short in delivering.
Take the example of cloud computing, a paradigm which allows users to outsource their data storage and computing needs to a powerful third party server such as Amazon. Though such a service is very useful, users may be reluctant to trust third party servers with sensitive data. Organizations utilizing these services must also ensure that their clients are secure from each other. At the same time, meaningful functionality must be provided. For example a server storing medical data might be required to grant users access to certain useful functions computed on the entire user database, such as the success rate of some medication for a given disease, while making sure that individual medical privacy is not compromised.
To address these emerging needs, a new paradigm of encryption was recently put forward. Functional Encryption. In functional encryption, a user’s secret key can be associated with its holder’s credentials, while the ciphertext can be associated with an access policy.We may ask that decryption succeed if and only if the credentials satisfy the access policy.
I will describe several special cases of functional encryption that we have constructed — systems for the identity function (identity based encryption or IBE), threshold function (fuzzy IBE) and linear functions. I will describe ongoing work to provide a general framework for these constructions and challenges faced in supporting more general functions.The technical tool we use in these constructions is the worst-case hardness of lattice problems. Lattices have traditionally been used in cryptography for breaking cryptosystems and their use in building cryptosystems is surprising and elegant.
To Read more: http://www.csa.iisc.ernet.in/sem-evts/seminars.php#top
Deformation, breakup and dielectrophoresis of drops under non-uniform electric fields
Abstract
A detailed non-linear analysis of the deformation and breakup of a perfect dielectric (PD) drop, suspended in another perfect dielectric fluid, in the presence of a quadrupole electric field is presented using analytical (asymptotic) and numerical (Boundary Integral) methods. The quadrupole field is the simplest kind of an axisymmetric non-uniform electric field. Several novel features are observed when compared to that of a drop under a uniform electric field. The first order analysis predicts oblate deformation for a PD-PD system when the dielectric constant of the suspending medium is larger than that of the drop. This is in contrast to uniform electric fields where oblate shapes are observed only in leaky dielectric (LD-LD) systems. Prolate shapes are observed when the drop is more polarisable, and the deformation is larger than that for uniform fields for similar electric capillary numbers. The steady state shapes are defined by several higher harmonics as compared to the uniform field. The second order analysis predicts larger prolate deformations but saturation of oblate shapes. At large capillary numbers, prolate deformations show breakup at all ratios of dielectric constants unlike uniform fields. Positive and negative dielectrophoresis is observed when the drop is placed off-center, and its translation and simultaneous deformation under quadrupole fields is also investigated. The electro-hydrostatics is unaffected by the viscosity ratio. However, the break-up of the drop and the dielectrophoretic motion and deformation strongly depend upon the viscosity ratio.
To Read more: http://chemeng.iisc.ernet.in/chem-webpage/seminars-2011-details.php#rochish
Dynamics of microconfined droplets in shear flow: Breakup and coalescence
Abstract
Recently, the ability to create structures on micron and smaller length scales has led to the utilization of microfluidic devices in a variety of applications. Multiphase flows provide several mechanisms for enhancing and extending the performance of single phase microfluidic systems. The properties of multiphase fluids are strongly dependent on their morphology, which is generated by a combination of droplet breakup, deformation, relaxation and coalescence. When transporting complex two-phasic fluids in microdevices, deviations from bulk behaviour can be expected if the dimensions of the channel become comparable to the size of the dispersed phase. In order to fully understand the underlying physics, confined shear flow is often chosen as a model flow condition for studying morphology development. In the present work, we focus on the effects of geometrical confinement on droplet breakup and coalescence in shear flow. The droplet dynamics have been visualized by means of a home-built counter rotating parallel plate shear flow cell with a microscopy setup. It was found that the effects of geometrical confinement on the critical conditions for droplet breakup are governed by material parameters such as viscosity ratio and component viscoelasticity. In addition, the breakup mode and resulting amount of daughter droplets are affected by these parameters. The experimental data on droplet breakup have been used to validate phenomenological models for droplet dynamics. In addition, numerical simulations with the Volume-Of-Fluid method have been used to interpret the observations in blends with viscoelastic components, showing that the viscoelastic stresses are significantly larger in confined as compared to bulk conditions. In addition to droplet breakup, also the critical conditions for droplet coalescence are affected by geometrical confinement. In this case, the relative position of the droplets with respect to each other, rather than the viscosity ratio, is essential for the effects of geometrical confinement. In order to obtain a better understanding of the forces and fluid dynamics that govern the interactions between sheared droplets, the droplet trajectories, the time-dependent orientation angles of the droplet doublets and the droplet deformation during the interaction process have systematically been studied.
To Read more: http://chemeng.iisc.ernet.in/chem-webpage/seminars-2011-details.php#ruth
