A novel vacuum ultra violet lamp for metastable rare gas experiments

We report on a new design of a vacuum ultra violet (VUV) lamp for direct optical excitation of high laying atomic states e.g. for excitation of metastable rare gas atoms. The lamp can be directly mounted to ultra high vacuum vessels (p <= 10^(-10) mbar). It is driven by a 2.45 GHz microwave source. For optimum operation it requires powers of approximately 20 W. The VUV light is transmitted through a magnesium fluoride window, which is known to have a decreasing transmittance for VUV photons with time. In our special setup, after a run-time of the VUV lamp of 550 h the detected signal continuously decreased to 25 % of its initial value. This corresponds to a lifetime increase of two orders of magnitude compared to previous setups or commercial lamps

Erosion patterns in a sediment layer

We report here on a laboratory-scale experiment which reproduces a rich variety of natural patterns with few control parameters. In particular, we focus on intriguing rhomboid structures often found on sandy shores and flats. We show that the standard views based on water surface waves come short to explain the phenomenon and we evidence a new mechanism based on a mud avalanche instability.Comment: 4 pages, 4 figures, to appear as Phys. Rev. E rapid com

Dip coating with colloids and evaporation

International audienceThere is a growing interest in coating hard and soft substrates with colloids, with numerous applications to optics and microelectronics [1]. A possibility to realize these substrates is to use dip coating with evaporation [2], i.e. to remove at constant speed a plate from a bath of colloids while drying occurs. This leads to several undesired effects: defects, heterogeneous deposition, fracture and de-lamination [1,3]. The problem is also difficult to model as three divergences may coexist at the contact line (CL) receding on the substrate [4-5] (and even in a advancing case [6]) : (1) divergence of viscous stresses, (2) divergence of evaporation as in the well known "coffee stain" effect [7-9], (3) and divergence of colloid concentration. In a recent paper we modeled the hydrodynamics in the vicinity of a moving, evaporating, contact line [4], and we found that in the dip coating case there should exist two different regimes at respectively low and high plate velocity, in which the deposed mean thickness should respectively decrease and increase with the plate velocity. This should lead to a minimum of the deposed thickness for a critical intermediate velocity. Up to a recent thesis in our group [5], this effect has never been evidenced in a dip coating experiment, though similar behaviors were found for deposition of phospholipids [10], and for colloids in a rather specific two-plate geometry (meniscus receding in a Hele-Shaw cell) [11-12]. We present here evidences in favour of this effect, revealed by this work, and we correct the model of ref.[4] which contained a mistake. A sketch of the experimental set up is suggested on Fig.1. A clean glass plate is plunged inside a colloidal suspension and removed from this bath at constant speed (V ranging between 50 µm/s and 5 cm/s), while deposition and evaporation takes place on the glass. We used silica suspensions (Klebosol silica sluries 50R50, 30R25 and 30R12) with three different particle diameters (12 nm, 25 nm and 50 nm), and two different volume concentrations (φ 0 =5% and 10%). The glass plate is cleaned and prepared before each experiment by the following protocole. First the glass surface is rub with a abrasive cerium oxid suspension (concentration 20%), cleaned with pure water, ethanol, and again pure water, and then let to dry. A plasma treatment is then imposed to the glass

A novel vacuum ultra violet lamp for metastable rare gas experiments

We report on a new design of a vacuum ultra violet (VUV) lamp for direct optical excitation of high laying atomic states e.g. for excitation of metastable rare gas atoms. The lamp can be directly mounted to ultra high vacuum vessels (p <= 10^(-10) mbar). It is driven by a 2.45 GHz microwave source. For optimum operation it requires powers of approximately 20 W. The VUV light is transmitted through a magnesium fluoride window, which is known to have a decreasing transmittance for VUV photons with time. In our special setup, after a run-time of the VUV lamp of 550 h the detected signal continuously decreased to 25 % of its initial value. This corresponds to a lifetime increase of two orders of magnitude compared to previous setups or commercial lamps

Dip coating with colloids and evaporation

International audienceThere is a growing interest in coating hard and soft substrates with colloids, with numerous applications to optics and microelectronics [1]. A possibility to realize these substrates is to use dip coating with evaporation [2], i.e. to remove at constant speed a plate from a bath of colloids while drying occurs. This leads to several undesired effects: defects, heterogeneous deposition, fracture and de-lamination [1,3]. The problem is also difficult to model as three divergences may coexist at the contact line (CL) receding on the substrate [4-5] (and even in a advancing case [6]) : (1) divergence of viscous stresses, (2) divergence of evaporation as in the well known "coffee stain" effect [7-9], (3) and divergence of colloid concentration. In a recent paper we modeled the hydrodynamics in the vicinity of a moving, evaporating, contact line [4], and we found that in the dip coating case there should exist two different regimes at respectively low and high plate velocity, in which the deposed mean thickness should respectively decrease and increase with the plate velocity. This should lead to a minimum of the deposed thickness for a critical intermediate velocity. Up to a recent thesis in our group [5], this effect has never been evidenced in a dip coating experiment, though similar behaviors were found for deposition of phospholipids [10], and for colloids in a rather specific two-plate geometry (meniscus receding in a Hele-Shaw cell) [11-12]. We present here evidences in favour of this effect, revealed by this work, and we correct the model of ref.[4] which contained a mistake. A sketch of the experimental set up is suggested on Fig.1. A clean glass plate is plunged inside a colloidal suspension and removed from this bath at constant speed (V ranging between 50 µm/s and 5 cm/s), while deposition and evaporation takes place on the glass. We used silica suspensions (Klebosol silica sluries 50R50, 30R25 and 30R12) with three different particle diameters (12 nm, 25 nm and 50 nm), and two different volume concentrations (φ 0 =5% and 10%). The glass plate is cleaned and prepared before each experiment by the following protocole. First the glass surface is rub with a abrasive cerium oxid suspension (concentration 20%), cleaned with pure water, ethanol, and again pure water, and then let to dry. A plasma treatment is then imposed to the glass