לוח שנה משולב

TBD

Abstract.
My talk will be devoted to my most interesting experiences in science and scientific computing. Three topics will be covered. First, I will talk about ill-posed inverse problems in astrophysics of cosmic dust. I will show how to formulate and solve a typical problem of modeling of interstellar dust by using a mathematically correct tool: the method of Tikhonov's regularization. Second, I will demonstrate how usually incomplete multi-sensor satellite-obtained data in Earth Sciences can be efficiently combined by using data fusion methods to produce complete global and regional data maps. Finally, I will discuss the theoretical grounds, practical implementation, and typical results derived with my polarized radiative transfer code PRT, which is of potential usefulness for atmospheric and astrophysical remote sensing applications.

Abstract
Despite its long history, several aspects of friction remain ill understood even today. This can be partly attributed to the fact that mechanisms contributing to friction are scale dependent. Further, several other factors such as the possible presence of interfacial layer between the contacting surfaces, plastic deformation of the contacting asperities, contact electrification etc., are also known to contribute. A decade ago, Budakian and Putterman (Phys. Rev. Lett., 85, 1000 (2000)) ascribed friction to the formation of bonds arising from contact charging when a gold tip of a surface force apparatus was dragged on polymethylmethacrylate (PMMA) surface. The authors demonstrate a correlation between stick-slip events and charge transfer when the gold tip of a Surface Force Apparatus was dragged with a velocity of a few μm/s on PMMA substrate. Typical measured charge density ~ 108 charges/mm2. The magnitude of the slip events is proportional to the ensuing charge transfer to the PMMA surface. The total force is equal to the total charge deposited over the scan length times a scale factor α. The value of α ~ 0.4 eV is close to the energy window for charge transfer between the surface states of PMMA and metallic Fermi level. Further, α was nearly constant for the range of normal loads from 68 to 106 mN. These intriguing results have not been explained so far.
Here we propose a model that is based on contact dynamics where plastic deformation of the interfacial material leads to slip. The equations of motion for the position and the depth of penetration of the gold tip are coupled to the equation for the contact charge density. Charging occurs during the stick phase and charge transfer to the PMMA substrate occurs during the slip phase. The stick-slip instability arises from a competition between the visco-elastic and plastic deformation time scales and, that due to the pull speed. First conclusion is that contact charging plays a minor role, a fact supported by earlier investigations on the effect of charging on adhesion. Our stick-slip model captures the observed correlation between stick-slip events and charge transfer, and the lack of dependence of the scale factor connecting the force jumps and charge transfer on normal load. It also recovers the value of α once the experimental value of charging radius is used from experiments. Thus, the model provides an alternate basis for explaining most experimental results without ascribing friction to contact charging.

The main-belt comets (MBCs) are a new class of objects, with asteroid-like orbits and cometary-like appearance. This is odd, since these have spent most of their lifetimes in the main belt, which has been considered too hot for ice to survive for any length of time. The possible prevalence of this population raises the intriguing possibility of a new and unexplored reservoir for water in the solar system. I will discuss the general framework of small icy body populations in the solar system and its relation to the MBCs, the observed and suggested properties of such a population and a suggested NASA space mission to characterize an MBC.