לוח שנה משולב

In a “standard model”, filamentation of femtosecond laser pulses in atmospheric-pressure gases results from dynamic balance of self-focusing via Kerr lensing and defocusing due to ionization-generated plasma. Regarding both of these effects, recent developments cast doubts on the basic model assumptions, which become overstretched as the filamenting laser pulse shortens and increases in intensity. Attempts at revising the role of higher-order Kerr effects lead to a controversy being currently hotly debated. The scenario of plasma emergence has also come under scrutiny. We discuss modifications of these basic effects on microscopic level and show how essentially new features arise in the nonlinear optical response of the medium and call for re-evaluation of the currently accepted picture of filament formation. Further, we consider nonlinear optics of the filament wake channels, which is indicative of the excited system evolution in the channel. Specifically, we discuss four-wave mixing in the nonequilibrium electron gas and dynamic Rabi sidebands originating from the excited atoms.

We showed years ago by fusing two differentiated cell types in stable non-dividing heterokaryons that “terminally” differentiated human cells could be reprogrammed. The balance of regulators was critical in determining the direction of differentiation. We are now enlisting natural mechanisms to tip the balance of regulators and derive new mammalian cell sources for regenerative medicine: (1) by using heterokaryons to identify crucial early regulators of reprogramming to pluripotency (iPS); (2) by altering telomerase activity; (3) by mimicking cues of adult stem cell niches; and (4) by dedifferentiation like newts. Our experimental systems offer a means to explore regulatory networks. Elucidation of the logic underlying nuclear reprogramming via molecular timelapse snapshots (the “Kineticon) is revealing discrete steps in pathways of dedifferentiation and redifferentiation. These approaches provide fundamental mechanistic insights and are revealing common principles of nuclear reprogramming. The generation of novel cell sources should enable new clinical applications of cell therapies for regenerative medicine.

All animals sleep, or do they? This question remains controversial. If sleep is truly universal to the animal kingdom then even the simplest model animal should sleep, and may offer valuable clues regarding the origin and core function of sleep. The nematode Caenorhabditis elegans develops through four larval stages before it reaches adulthood. At the transition between stages and before it molts, i.e., synthesizes a new exoskeleton and sheds the old one, it exhibits a quiescent state termed lethargus. In a seminal paper in 2008, David Raizen has demonstrated that lethargus bears several similarities to sleep. The talk will focus on behavioral aspects of lethargus and establishing C. elegans as a model system for sleep. Examples of behavioral dynamics associated with lethargus include the nematode’s hockey stick-like posture and its hypothesized functionality, non-Markovian locomotion/quiescence dynamics (micro-homeostasis), responses to external stimuli that exhibit sensory gating, and the onset and timing of quiet wakefulness. As time permits, neurophysiological and genetic aspects of worm-sleep will be briefly discussed.

Solids brought in contact with an electrolyte spontaneously acquire surface charge, e.g. via ionization/dissociation of surface groups. A
balance between electrostatic forces and diffusion leads to the formation of a screening (Debye) layer where counter-ions are in excess and
co-ions are in deficit; the Debye length, on which space-charge density decays towards the electro-neutral bulk, is typically no more than a
few tens of nanometers. When exposed to an external electric field, the Debye layer is sheared in response to Coulomb body forces acting on
the charged liquid. On a scale much larger than the Debye length, this effect is manifested as "electro-osmotic slip". Thus, a freely suspended
micron-sized particle will migrate electrophoretically in response to the effective slip distribution induced over its surface — notwithstanding
the net electro-neutrality of the particle considered together with the Debye layer surrounding it.
From a modeling point of view, mutual coupling between ionic transport, electrostatics, surface chemistry, and hydrodynamics leads to a
mathematical formulation which is highly nonlinear. Moreover, the extreme scale disparity associated with the thin-Debye-layer limit hinders
the application of standard numerical methods. Ever since the intuitive derivation of the time-honored Smoluchowski slip condition (the
domain of validity of which is not always evident), most analyses have employed various linearizations tantamount to assuming weak applied
fields or small surface-charge densities. In many cases of practical interest, however, these assumptions are simply inadequate. In this talk, I
will describe how a simplified coarse-grained model – valid for arbitrary surface charge density and field strength – can be systematically
derived by exploiting the above-mentioned scale disparity. Approximate analyses of these models, complemented by numerical simulations on
the macroscale, will also be presented. These allow for an intuitive grasp along with quantitative predictions. Finally, the electrophoretic
migration of drops and bubbles will be considered following a similar thin-Debye-layer methodology. A unique mechanism for electrokinetic
flow is unraveled in the case of a gas bubble thereby resolving a long-lasting paradox.
Joint work with Ehud Yariv and Itzchak Frankel.