Waterjet and laser etching: the nonlinear inverse problem.

In waterjet and laser milling, material is removed from a solid surface in a succession of layers to create a new shape, in a depth-controlled manner. The inverse problem consists of defining the control parameters, in particular, the two-dimensional beam path, to arrive at a prescribed freeform surface. Waterjet milling (WJM) and pulsed laser ablation (PLA) are studied in this paper, since a generic nonlinear material removal model is appropriate for both of these processes. The inverse problem is usually solved for this kind of process by simply controlling dwell time in proportion to the required depth of milling at a sequence of pixels on the surface. However, this approach is only valid when shallow surfaces are etched, since it does not take into account either the footprint of the beam or its overlapping on successive passes. A discrete adjoint algorithm is proposed in this paper to improve the solution. Nonlinear effects and non-straight passes are included in the optimization, while the calculation of the Jacobian matrix does not require large computation times. Several tests are performed to validate the proposed method and the results show that tracking error is reduced typically by a factor of two in comparison to the pixel-by-pixel approach and the classical raster path strategy with straight passes. The tracking error can be as low as 2-5% and 1-2% for WJM and PLA, respectively, depending on the complexity of the target surface.

Stochastic simplified modelling of abrasive waterjet footprints.

Abrasive micro-waterjet processing is a non-conventional machining method that can be used to manufacture complex shapes in difficult-to-cut materials. Predicting the effect of the jet on the surface for a given set of machine parameters is a key element of controlling the process. However, the noise of the process is significant, making it difficult to design reliable jet-path strategies that produce good quality parts via controlled-depth milling. The process is highly unstable and has a strong random component that can affect the quality of the workpiece, especially in the case of controlled-depth milling. This study describes a method to predict the variability of the jet footprint for different jet feed speeds. A stochastic partial differential equation is used to describe the etched surface as the jet is moved over it, assuming that the erosion process can be divided into two main components: a deterministic part that corresponds to the average erosion of the jet and a stochastic part that accounts for the noise generated at different stages of the process. The model predicts the variability of the trench profiles to within less than 8%. These advances could enable abrasive micro-waterjet technology to be a suitable technology for controlled-depth milling.

Thin three-dimensional droplets on an oscillating substrate with contact angle hysteresis.

Recent experiments [P. Brunet, J. Eggers, and R. D. Deegan, Phys. Rev. Lett. 99, 144501 (2007)10.1103/PhysRevLett.99.144501] have shown that a liquid droplet on an inclined plane can be made to move uphill by sufficiently strong, vertical oscillations. In order to investigate this counterintuitive phenomenon we use a model in which liquid inertia and viscosity are assumed negligible so that the motion of the droplet is dominated by the applied acceleration due to the oscillation of the plate, gravity, and surface tension. We explain how the leading order motion of the droplet can be separated into a spreading mode and a swaying mode. For a linear contact line law, the maximum rise velocity occurs when these modes are in phase. We show that, both with and without contact angle hysteresis, the droplet can climb uphill and also that, for certain contact line laws, the motion of the droplet can produce footprints similar to experimental results. We show that if the two modes are out of phase when there is no contact angle hysteresis, the inclusion of hysteresis can force them into phase. This in turn increases the rise velocity of the droplet and can, in some cases, cause a sliding droplet to climb.

Three-dimensional elastohydrodynamics of a thin plate oscillating above a wall.

We consider deflections of a thin rectangular elastic plate that is submerged within a Newtonian fluid. The plate is clamped along one edge and supported horizontally over a plane horizontal wall. We consider both external driving, where the clamped edge is vibrated vertically at high frequencies, and thermal driving, where the plate fluctuates under Brownian motion. In both cases, the amplitude of oscillation is assumed sufficiently small that the resulting flow has little convective inertia, although the oscillation frequency is sufficiently high to generate substantial unsteady inertia in the flow, a common scenario in many nano- and microdevices. We exploit the plate's thinness to develop an integral-equation representation for the three-dimensional flow (a so-called thin-plate theory) which offers considerable computational savings over a full boundary-integral formulation. Limiting cases of high oscillation frequencies and small wall-plate separation distances are studied separately, leading to further simplified descriptions for the hydrodynamics. We validate these reduced integral representations against full boundary-integral computations, and identify the parameter ranges over which these simplified formulations are valid. Addressing the full flow-structure interaction, we also examine the limits of simpler two-dimensional hydrodynamic models. We compare the responses of a narrow plate under two- and three-dimensional hydrodynamic loading, and report differences in the frequency response curves that occur when the plate operates in water, in contrast to the excellent agreement observed in air.

Stochastic elastohydrodynamics of a microcantilever oscillating near a wall.

We consider the thermally driven motion of a microcantilever in a fluid environment near a wall, a configuration characteristic of the atomic force microscope. A theoretical model is presented which accounts for hydrodynamic interactions between the cantilever and wall over a wide range of frequencies and which exploits the fluctuation-dissipation theorem to capture the Brownian dynamics of the coupled fluid-cantilever system. Model predictions are tested against experimental thermal spectra for a cantilever in air and water. The model shows how, in a liquid environment, the effects of non-delta-correlated Brownian forcing appear in the power spectrum, particularly at low frequencies. The model also predicts accurately changes in the spectrum in liquid arising through hydrodynamic wall effects, which we show are strongly mediated by the angle at which the cantilever is tilted relative to the wall.

A model of in-vivo hydrocephalus shunt dynamics for blockage and performance diagnostics.

The accumulation of excess cerebrospinal fluid in the ventricles of the brain results in hydrocephalus, a condition that is fatal if left untreated. The usual remedy is to insert a shunt into the ventricles of the brain, which drains excess fluid away, moderated by a pressure dependent valve. It is important that the system functions properly so that a reasonable intracranial pressure is maintained. Unfortunately, pressure measurements in the ventricles are highly invasive, while pressure measurements in the shunt outside the skull may not detect any blockage in the catheter inside. Here we develop a model primarily aimed at detecting in vivo a blockage and other shunt malfunction using non-invasive measurements, so that shunt valves can be adjusted accordingly. The system offers a clear insight into how currently available clinical measurements may be utilized. We then extend this to investigate the phenomenon of 'chatter' (rapid opening and closing) and other mechanisms including intracranial pressure pulsatility. Although simple, the model offers a clear indication of what is required for successful regulation of both intracranial pressure and shunt flow.

Adsorption of Polycations on Clays: A Comparative in situ Study Using 133Cs and 23Na Solution Phase NMR

23Na solution phase NMR has been evaluated as an in situ probe to study the adsorption of tetramethylammonium (TMA+) and two polycations, FL17 ([(Me2NCH2CHOHCH2)n]n+Cln) and Magnafloc 1697, ([(CH2CHCH2N(Me)2CH2&Cmacr;HCH2)n]n+Cln), onto clays in aqueous suspensions containing 2.5 mass% low iron Texas bentonite. The NMR data shows the effectiveness of the organocations at displacing Na+ from the bentonite surface. This information has been correlated with that obtained from particle-size and electrophoretic measurements in aqueous solution, together with information from adsorption isotherms. These results have been compared to those obtained in parallel studies using 133Cs solution phase NMR. FL17 and 1697 both exhibited high affinity adsorption isotherms on Na+- and Cs+-clay, whereas the adsorption of TMA+, which represents the cationic portion of the polymers was of lower affinity. Na+-bentonite adsorbed almost twice the amount of polycation required to fulfill the cation-exchange capacity (CEC) of the bentonite. The electrophoretic and particle size data indicated significant differences in the size of the polycation/clay flocs and the amount of polymer adsorbed on the external faces of the flocs in the presence of Na+- and Cs+-exchange ions. Correlation of this data with the NMR results suggests that the Na+-bentonite/polycation flocs are large, of low density, and that the polycation is concentrated in the interior while the Na+-ions occupy exchange sites on the external faces. Copyright 1997 Academic Press. Copyright 1997Academic Press

Detection of the Earth with the SETI microwave observing system assumed to be operating out in the galaxy.

This paper estimates the maximum range at which radar signals from the Earth could be detected by a search system similar to the NASA Search for Extraterrestrial Intelligence Microwave Observing Project (SETI MOP) assumed to be operating out in the galaxy. Figures are calculated for the Targeted Search, and for the Sky Survey parts of the MOP, both operating, as currently planned, in the second half of the decade of the 1990s. Only the most powerful terrestrial transmitters are considered, namely, the planetary radar at Arecibo in Puerto Rico, and the ballistic missile early warning systems (BMEWS). In each case the probabilities of detection over the life of the MOP are also calculated. The calculation assumes that we are only in the eavesdropping mode. Transmissions intended to be detected by SETI systems are likely to be much stronger and would of course be found with higher probability to a greater range. Also, it is assumed that the transmitting civilization is at the same level of technological evolution as ours on Earth. This is very improbable. If we were to detect another technological civilization, it would, on statistical grounds, be much older than we are and might well have much more powerful transmitters. Both factors would make detection by the NASA MOP a much more likely outcome.

A reply from Earth?

An approach is proposed to developing a message from Humankind to extraterrestrial intelligence if we detect it.

Risk and value analysis of SETI.

This paper attempts to apply a traditional risk and value analysis to the Search for Extraterrestrial Intelligence--SETI. In view of the difficulties of assessing the probability of success, a comparison is made between SETI and a previous search for extraterrestrial life, the biological component of Project Viking. Our application of simple Utility Theory, given some reasonable assumptions, suggests that SETI is at least as worthwhile as the biological experiment on Viking.

The evolution of complex life.

In considering the probabilities that intelligent life might exist elsewhere in the Universe, it is important to ask questions about the factors governing the emergence of complex living organisms in the context of evolutionary biology, planetary environments and events in space. Two important problems arise. First, what can be learned about the general laws governing the evolution of complex life anywhere in space by studying its history on the Earth? Second, how is the evolution of complex life affected by events in space? To address these problems, a series of Science Workshops on the Evolution of Complex Life was held at the Ames Research Center. Included in this paper are highlights of those workshops, with particular emphasis on the first question, namely the evolution of complex extraterrestrial life.

Review of methodology and technology available for the detection of extrasolar planetary systems.

Anyone undertaking an interstellar voyage might wish to be assured of the existence of a safe planetary harbor at the other end! Aside from the obvious interest of the participants in this Symposium, astronomers and astrophysicists are also eager to detect and study other planetary systems in order to better understand the formation of our own Solar System. Scientists involved in the search for extraterrestrial intelligence argue that planets suitable for the evolution of life may abound elsewhere within our own Milky Way Galaxy. On theoretical grounds, they are probably correct, but they lack any observational support. For in spite of decades of claimed astrometric detections of planetary companions and the recent exciting and tantalizing observations from the IRAS satellite and the IR speckle observations of Van Biesbroeck 8 and other cool stars, there is no unambiguous proof for the existence of another planetary system beyond our own. In this paper we review the various methods for detecting extrasolar planets and briefly describe the Earth and space based technology currently available and discuss the near-term plans to implement these different search techniques. In each case an attempt is made to identify the limiting source of systematic error inherent to the methodology and to assess the potential for technological improvements.

Baroreceptor function in the hypertensive black African.

Hypertension in the black African differs in some respects from white Europeans: complications due to accelerated atherosclerosis are rare and treatment with beta-blockers alone is ineffective. It is not known if baroreceptor function is depressed in African hypertensives to the same extent as it is in whites. Therefore, we have assessed baroreceptor reflex sensitivity (BRS) by the phenylephrine method in 19 African hypertensive patients living in the Gambia, West Africa. The results were compared to predicted BRS values for white patients of the same age and blood pressure calculated from a regression equation derived from 61 hypertensive patients studied in Oxford. It was found that baroreceptor reflex sensitivity was reduced in the African hypertensives and the log mean BRS was similar to the predicted value for Europeans of the same age and level of blood pressure (0.473 +/- 0.24 msec/mmHg and 0.489 +/- 0.21 msec/mmHg respectively). The resting mean arterial pressure in the African patients varied from 117 to 194 mmHg. The results indicated that African hypertensives have a depression of baroreflex sensitivity which is similar to European hypertensive patients.

Campylobacter enteritis in The Gambia.

In a three-month survey in The Gambia, campylobacters were isolated from 14.3% of 287 patients suffering from diarrhoea and from 4.2% of 383 patients and controls without diarrhoea. The equivalent figures for shigellas were 6.3% and 2.1%, and for salmonella 7.0% and 2.6% respectively. 53 (93%) of the 57 campylobacters were isolated from children less than five years old; none were isolated from indigenous adults (greater than 15 years old). By using two selective media in parallel (Butzler's medium and Skirrow's medium), it was found that the number of isolations was increased by one third over that obtained with either medium alone. It is concluded that in The Gambia the transmission of campylobacters is of high intensity, with most infections occurring in young children, in whom these organisms are an important cause of morbidity and mortality.

A comparison of two media for the isolation of campylobacter in the tropics.

Two selective media for the isolation of campylobacter from the faeces have been compared. Both have a similar isolation rate, but Butzler's medium allows easier identification and sub-culturing of campylobacter than Skirrow's medium because it is more inhibitory to other faecal organisms. Butzler's medium is made with sheep blood which is usually more easily available in Africa, so for these reasons it may be a more suitable medium for use there.

Motion sickness and otolith sensitivity: a pilot study of habituation to linear acceleration.

Astronauts, particularly in Skylab flights, experienced varying degrees of motion sickness lasting 3-5 d. One possible mechanism for this motion sickness adaptation is believed to be a reduction in otolith sensitivity with an attendant reduction in sensory conflict. In an attempt to determine if this hypothesis is valid, a ground-based pilot study was conducted on a vertical linear accelerator. The extent of habituation to accelerations which initially produced motion sickness was evaluated, along with the possible value of habituation training to minimize the space motion sickness problem. Results showed that habituation occurred for 6 of the 8 subjects tested. However, in tests designed to measure dynamic and static otolith function, no significant differences between pre- and post-habituation tests were observed. Cross habituation effects to a standard Coriolis acceleration test were not significant. It is unlikely that ground-based pre-habituation to linear accelerations of the type examined would alter susceptibility to space motion sickness.

Effect of altered G levels on deposition of particulates in the human respiratory tract.

The primary mode of depositon of particles in the respiratory tract in the size range 0.5-10 mum diam (unit density) is sedimentation. The rate of sedimentation is directly proportional to the velocity of settling of the particle. Therefore, the total deposition of particles in the respiratory tract as well as the region of deposition is affected by changes in gravity. Human subjects were exposed to aerosols of 2.02-mum-diam polyvinyltoluene particles at 0, 0.5, 1.0, and 2.0 G. Total deposition was measured at each G level. Results indicate an almost linear increase in total deposition with increasing G levels over the range studied. The deposition measured at 1 G was less than reported in earlier experiments and the deposition at levels less than 1 G was less than had been calculated by Muir and Beeckmans. These data show that although sedimentation plays the major role in depostion of 2.02 mum particles, it is less than previously described.

The effects of cosmic particle radiation on pocket mice aboard Apollo XVII: X. Results of ear examination.

In the five pocket mice flown on Apollo XVII, no evidence was found that the inner ear had been damaged, though poor fixation precluded detailed study. On the other hand, the middle ear cavity was involved in all the mice, hemorrhage having occurrred in response to excursions in pressure within the canister that housed the mice during their flight. The same occurred in flight control mice which had been subjected to pressure excursions of much the same magnitude. A greater degree of exudation into air cells and greater leukotaxis were noted in the flight animals than in the control animals. There was no increase in leukocyte population along the paths of the 23 cosmic ray particles registered in the subscalp dosimeters that traversed the middle ear cavities of the flight mice. The increased exudation and the greater response by leukocytes in the flight mice may have been causally related to the lesions found in their olfactory mucosa but there were no data in support of this possibility.