Bitumen surface microstructure evolution in subzero environments.

Bitumen is a widely used material employed as a binder in pavement engineering and as a surface sealant in construction. Its surface microstructure and microscale properties have been shown to be temperature-dependent, with effects manifesting themselves on surface composition and texture, including the formation of the visually striking catana 'bee'-like structures. Despite the importance of a good performance of bitumen in subzero environments (<0°C), the behaviour of bitumen surface texture and composition at cold temperatures, affecting cracking, degradation and road icing, has received practically no attention. In particular, such knowledge is relevant to world regions experiencing long periods of subzero temperatures during the year. Employing advanced atomic force microscopy combined with infrared spectroscopy (AFM-IR) and an environmental chamber, we demonstrate the ability to characterise surface structure and composition with nanoscale precision for a broad range of temperatures. We show that cooling bitumen to subzero temperatures can have several interesting effects on its surface microtexture, nanotexture and composition, especially on its three surface domains, catana, peri and para. We found that the para domain coarsens and extends to form an interfacial transition domain (characterised by increasing surface roughness with peri domain composition) between the para and peri domains. We show that the catana and peri domains have a similar composition, but have different mechanical and chemical properties compared to the para domain. The essential findings of this work improve our understanding of the behaviour of bitumen in subzero environments, aiding us in our quest towards attaining better road and sealant performance.

Sub-micron lateral topography affects endothelial migration by modulation of focal adhesion dynamics.

Through the interaction with topographical features, endothelial cells tune their ability to populate target substrates, both in vivo and in vitro. Basal textures interfere with the establishment and maturation of focal adhesions (FAs) thus inducing specific cell-polarization patterns and regulating a plethora of cell activities that govern the overall endothelial function. In this study, we analyze the effect of topographical features on FAs in primary human endothelial cells. Reported data demonstrate a functional link between FA dynamics and cell polarization and spreading on structured substrates presenting variable lateral feature size. Our results reveal that gratings with 2 µm lateral periodicity maximize contact guidance. The effect is linked to the dynamical state of FAs. We argue that these results are readily applicable to the rational design of active surfaces at the interface with the blood stream.

Repolarisation descriptors and heart rate variability in hemodialysed patients.

T wave morphology (TWM) descriptors derived from Holter electrocardiograms during hemodialysis (HD) are of potential value for cardiac risk assessment in HD patients. Our knowledge on autonomic regulation of TWM descriptors is limited. The purpose of this study was to investigate the association between TWM parameters and heart rate variability (HRV) during intradialytic monitoring. In each of 81 patients on maintenance HD, continuous electrocardiograms were recorded 5 times during HD on alternate weeks. TWM descriptors were calculated every 5 s in overlapping 10-s ECG segments and Low Frequency (LF) (0.04 Hz to 0.15 Hz), High Frequency (HF) (0.15 Hz to 0.40 Hz) powers of the spectrum of HRV were calculated every five min. The calculated values of TWM and HRV were averaged during the first hour of the recordings and subsequently over all recordings in each subject. Analyzable data for HRV and TWM were available in 71 HD patients (aged 61+/-15, 36 % diabetics, 32 % females). LF in normalized units correlated positively with Total Cosine R to T (r=0.374, p=0.001) and negatively with T wave morphology dispersion (r=-0.253, p=0.033) after adjusting for heart rate. A heart rate independent association between repolarisation descriptors and HRV exists in HD patients. Autonomic modulation needs to be considered when using TWM characteristics for risk profiling of HD patients.

Unraveling wetting transition through surface textures with X-rays: liquid meniscus penetration phenomena.

In this report we show that synchrotron X-ray radiography is a powerful method to study liquid-air interface penetration through opaque microtextured surface roughness, leading to wetting transition. We investigate this wetting phenomenon in the context of sessile drop evaporation, and establish that liquid interface sinking into the surface texture is indeed dictated by the balance of capillary and Laplace pressures, where the intrinsically three-dimensional nature of the meniscus must be accounted for. Air bubble entrapment in the texture underneath impacting water drops is also visualized and the mechanisms of post-impact drop evaporation are discussed.

Water drops dancing on ice: how sublimation leads to drop rebound.

Drop rebound is a spectacular event that appears after impact on hydrophobic or superhydrophobic surfaces but can also be induced through the so-called Leidenfrost effect. Here we demonstrate that drop rebound can also originate from another physical phenomenon, the solid substrate sublimation. Through drop impact experiments on a superhydrophobic surface, a hot plate, and solid carbon dioxide (commonly known as dry ice), we compare drop rebound based on three different physical mechanisms, which apparently share nothing in common (superhydrophobicity, evaporation, and sublimation), but lead to the same rebound phenomenon in an extremely wide temperature range, from 300 °C down to even below -79 °C. The formation and unprecedented visualization of an air vortex ring around an impacting drop are also reported.

Syngas generation from n-butane with an integrated MEMS assembly for gas processing in micro-solid oxide fuel cell systems.

An integrated system of a microreformer and a carrier allowing for syngas generation from liquefied petroleum gas (LPG) for micro-SOFC application is discussed. The microreformer with an overall size of 12.7 mm × 12.7 mm × 1.9 mm is fabricated with micro-electro-mechanical system (MEMS) technologies. As a catalyst, a special foam-like material made from ceria-zirconia nanoparticles doped with rhodium is used to fill the reformer cavity of 58.5 mm(3). The microreformer is fixed onto a microfabricated structure with built-in fluidic channels and integrated heaters, the so-called functional carrier. It allows for thermal decoupling of the cold inlet gas and the hot fuel processing zone. Two methods for heating the microreformer are compared in this study: a) heating in an external furnace and b) heating with the two built-in heaters on the functional carrier. With both methods, high butane conversion rates of 74%-85% are obtained at around 550 °C. In addition, high hydrogen and carbon monoxide yields and selectivities are achieved. The results confirm those from classical lab reformers built without MEMS technology (N. Hotz et al., Chem. Eng. Sci., 2008, 63, 5193; N. Hotz et al., Appl. Catal., B, 2007, 73, 336). The material combinations and processing techniques enable syngas production with the present MEMS based microreformer with high performance for temperatures up to 700 °C. The functional carrier is the basis for a new platform, which can integrate the micro-SOFC membranes and the gas processing unit as subsystem of an entire micro-SOFC system.

Direct printing of nanostructures by electrostatic autofocussing of ink nanodroplets.

Nanotechnology, with its broad impact on societally relevant applications, relies heavily on the availability of accessible nanofabrication methods. Even though a host of such techniques exists, the flexible, inexpensive, on-demand and scalable fabrication of functional nanostructures remains largely elusive. Here we present a method involving nanoscale electrohydrodynamic ink-jet printing that may significantly contribute in this direction. A combination of nanoscopic placement precision, soft-landing fluid dynamics, rapid solvent vapourization, and subsequent self-assembly of the ink colloidal content leads to the formation of scaffolds with base diameters equal to that of a single ejected nanodroplet. The virtually material-independent growth of nanostructures into the third dimension is then governed by an autofocussing phenomenon caused by local electrostatic field enhancement, resulting in large aspect ratio. We demonstrate the capabilities of our electrohydrodynamic printing technique with several examples, including the fabrication of plasmonic nanoantennas with features sizes down to 50 nm.

On ultrasound-induced microbubble oscillation in a capillary blood vessel and its implications for the blood-brain barrier.

The complex interaction between an ultrasound-driven microbubble and an enclosing capillary microvessel is investigated by means of a coupled, multidomain numerical model using the finite volume formulation. This system is of interest in the study of transient blood-brain barrier disruption (BBBD) for drug delivery applications. The compliant vessel structure is incorporated explicitly as a distinct domain described by a dedicated physical model. Red blood cells (RBCs) are taken into account as elastic solids in the blood plasma. We report the temporal and spatial development of transmural pressure (P(tm)) and wall shear stress (WSS) at the luminal endothelial interface, both of which are candidates for the yet unknown mediator of BBBD. The explicit introduction of RBCs shapes the P(tm) and WSS distributions and their derivatives markedly. While the peak values of these mechanical wall parameters are not affected considerably by the presence of RBCs, a pronounced increase in their spatial gradients is observed compared to a configuration with blood plasma alone. The novelty of our work lies in the explicit treatment of the vessel wall, and in the modelling of blood as a composite fluid, which we show to be relevant for the mechanical processes at the endothelium.

A case of recurrent benign lymphocytic (Mollaret's) meningitis and review of the literature.

Mollaret's meningitis is a rare form of benign recurrent aseptic meningitis first described in 1944. We report a case of Mollaret's meningitis due to Herpes Simplex Virus type 2 (HSV2), diagnosed with Polymerase Chain Reaction (PCR) implementation in the Cerebrospinal fluid (CSF) of the patient and treated successfully with acyclovir. To our knowledge, this is the first case of Mollaret's meningitis reported in Greece. We reviewed the literature since PCR has become widely available. Herpes Simplex Virus type 2 has been the most commonly identified causative agent of Mollaret's meningitis.

Focused ion beam-assisted manipulation of single and double beta-SiC nanowires and their thermal conductivity measurements by the four-point-probe 3-omega method.

Control of one-dimensional (1D) nanostructures is demonstrated in this paper by selectively placing and aligning silicon carbide (beta-SiC) nanowires (NWs). We developed a reliable and highly reproducible way of placing a single or double SiC NW on pre-patterned electrodes by using a focused ion beam and a nanomanipulator. 3-omega signals obtained by the four-point-probe method were used in measuring the thermal conductivity of the NWs. The thermal conductivities of the placed single and double beta-SiC NWs were obtained at 82 +/- 6 W mK( - 1) and 73 +/- 5 W mK( - 1), respectively. The proposed technique offers new possibilities for manipulating and evaluating 1D nanoscale materials.

The role of the carotid sinus in the reduction of arterial wall stresses due to head movements--potential implications for cervical artery dissection.

Spontaneous dissection of the cervical internal carotid artery (sICAD) is a major cause of stroke in young adults. A tear in the inner part of the vessel wall triggers sICAD as it allows the blood to enter the wall and develop a transmural hematoma. The etiology of the tear is unknown but many patients with sICAD report an initiating trivial trauma. We thus hypothesised that the site of the tear might correspond with the location of maximal stress in the carotid wall. Carotid artery geometries segmented from magnetic resonance images of a healthy subject at different static head positions were used to define a path of motion and deformation of the right cervical internal carotid artery (ICA). Maximum head rotation to the left and rotation to the left combined with hyperextension of the neck were investigated using a structural finite element model. A role of the carotid sinus as a geometrically compliant feature accommodating extension of the artery is shown. At the extreme range of the movements, the geometrical compliance of the carotid sinus is limited and significant stress concentrations appear just distal to the sinus with peak stresses at the internal wall on the posterior side of the vessel following maximum head rotation and on the anteromedial portion of the vessel wall following rotation and hyperextension. Clinically, the location of sICAD initiation is 10-30 mm distal to the origin of the cervical ICA, which corresponds with the peak stress locations observed in the model, thus supporting trivial trauma from natural head movements as a possible initiating factor in sICAD.

On the influence of variation in haemodynamic conditions on the generation and growth of cerebral aneurysms and atherogenesis: a computational model.

A risk-factor criterion, based on near-wall haemodynamic conditions, for the assessment of vascular pathology risk is developed and tested. This criterion has its foundation on experimentally observed vascular wall responses to oscillatory and swirling wall shear stress patterns and is applied to the results of computational simulations. We test this model on two anatomically accurate vascular segments, where pathologies are either commonplace or have already been developed, i.e. a healthy carotid bifurcation and a cerebral fusiform aneurysm. In the case of the former, the risk-assessment criterion predicts the emergence of atherosclerosis of the same locations that the disease is usually encountered. In the case of the latter, the risk factor shows increased probability for the appearance of secondary, "baby", aneurysms at certain locations.

Haemodynamics and wall remodelling of a growing cerebral aneurysm: a computational model.

We have developed a computational simulation model for investigating an often postulated hypothesis connected with aneurysm growth. This hypothesis involves a combination of two parallel and interconnected mechanisms: according to the first mechanism, an endothelium-originating and wall shear stress-driven apoptotic behavior of smooth muscle cells, leading to loss of vascular tone is believed to be important to the aneurysm behavior. Vascular tone refers to the degree of constriction experienced by a blood vessel relative to its maximally dilated state. All resistance and capacitance vessels under basal conditions exhibit some degree of smooth muscle contraction that determines the diameter, and hence tone, of the vessel. The second mechanism is connected to the arterial wall remodeling. Remodeling of the arterial wall under constant tension is a biomechanical process of rupture, degradation and reconstruction of the medial elastin and collagen fibers. In order to investigate these two mechanisms within a computationally tractable framework, we devise mechanical analogues that involve three-dimensional haemodynamics, yielding estimates of the wall shear stress and pressure fields and a quasi-steady approach for the apoptosis and remodeling of the wall. These analogues are guided by experimental information for the connection of stimuli to responses at a cellular level, properly averaged over volumes or surfaces. The model predicts aneurysm growth and can attribute specific roles to the two mechanisms involved: the smooth muscle cell-related loss of tone is important to the initiation of aneurysm growth, but cannot account alone for the formation of fully grown sacks; the fiber-related remodeling is pivotal for the latter.

Phase change of a confined subcooled simple liquid in a nanoscale cavity.

The phase transition of a simple liquid bounded between two parallel walls a few nanometers apart is investigated with molecular dynamics simulations. Vapor nucleation in a liquid confined in a microchannel of only a few nanometers in size cannot be achieved by increasing the temperature at the wall. Already small changes in temperature cause a large rise in pressure, in terms of orders of magnitude. On the other hand, using the fact that some fluids thermally contract on cooling, e.g., the argon liquid investigated here, reducing the temperature places the fluid in the liquid-vapor coexistence regime. If the bottom wall temperature is further reduced, the fluid will crystallize starting from the bottom surface, creating a "frozen" bubble in the crystallized state. It was found that the confining walls and not the quenching rate primarily affect the crystallization process. However, the fastest cooling rate investigated herein led to a decrease of the boiling initiation temperature. At a lower cooling rate, the vapor nucleation temperature was the same as the equilibrium boiling temperature for the confined liquid. Small temperatures in the confined system result in dominating attraction forces at the fluid-wall interface exposing the fluid to tensile stress. The increased influence of the walls results in a significant decrease of the boiling as well as freezing temperatures.

Solidification of gold nanoparticles in carbon nanotubes.

The structure and the solidification of gold nanoparticles in a carbon nanotube are investigated using molecular dynamics simulations. The simulations indicate that the predicted solidification temperature of the enclosed particle is lower than its bulk counterpart, but higher than that observed for clusters placed in vacuum. A comparison with a phenomenological model indicates that, in the considered range of tube radii (R(CNT)) of 0.5 < R(CNT) < 1.6 nm, the solidification temperature depends mainly on the length of the particle with a minor dependence on R(CNT).

Oscillatory behavior of nanodroplets.

Molecular dynamics (MD) simulations were performed in order to investigate the phenomenon of free oscillations of nanodroplets and the extent to which the continuum theory for such oscillations holds at nanoscales. The effect of temperature on these oscillations is also studied. The surface tension, a key property for the phenomenon of interest, was evaluated and compared with the experimental values of argon, showing that with an appropriate choice of the cutoff distance in the MD simulations, it is possible to predict the surface tension with good approximation. Nanoscale capillary waves on the free surface of the droplet were observed and compared to continuum theoretical predictions of the same. The nanodroplet interface thickness calculated based on continuum theory for these waves agreed well with the molecular dynamics calculation of the interface thickness. The frequencies of the oscillation of the droplet were calculated for all the studied temperatures and compared with the classical continuum theory. Although the simulated system cannot be considered strictly as a continuum, a good overall agreement was found.

Residence times and basins of attraction for a realistic right internal carotid artery with two aneurysms.

We are presenting computational fluid dynamics simulation results for the flow in an anatomically accurate right internal carotid artery, exhibiting two saccular aneurysms close to each other. Our study focuses on the investigation of passage times for blood cells through the two-aneurysm malformation. We construct residence time maps that exhibit strong non-uniformity, linked to the entry of fluid in only the first, only the second, or in both aneurysms. An entrance index is computed, showing qualitatively the regions at an arterial section upstream of the aneurysms, where cells following one of these scenarios emanate. The significance of the residence time profiles and entry scenarios obtained is discussed with respect to thrombosis and pharmacokinetics. Preliminary evidence that the inflow-outflow patterns of the two aneurysms may be leading to particularly complex flow and to chaotic mixing is discussed.