Evaporation-triggered segregation of sessile binary droplets

Droplet evaporation of multicomponent droplets is essential for various physiochemical applications, e.g. in inkjet printing, spray cooling and microfabrication. In this work, we observe and study phase segregation of an evaporating sessile binary droplet, consisting of a mixture of water and a surfactant-like liquid (1,2-hexanediol). The phase segregation (i.e., demixing) leads to a reduced water evaporation rate of the droplet and eventually the evaporation process ceases due to shielding of the water by the non-volatile 1,2-hexanediol. Visualizations of the flow field by particle image velocimetry and numerical simulations reveal that the timescale of water evaporation at the droplet rim is faster than that of the Marangoni flow, which originates from the surface tension difference between water and 1,2-hexanediol, eventually leading to segregation

Long-range elastic guidance mechanisms for electrostatic comb-drive actuators

The range of motion and output force of the often used electrostatic comb-drive with folded flexure straight guidance, as shown in Figure 1, is limited by sideways instability due to poor sideways stiffness of the folded flexure at relatively large deflections [1]

Clustering of vertically constrained passive particles in homogeneous, isotropic turbulence

We analyze the dynamics of small particles vertically confined, by means of a linear restoring force, to move within a horizontal fluid slab in a three-dimensional (3D) homogeneous isotropic turbulent velocity field. The model that we introduce and study is possibly the simplest description for the dynamics of small aquatic organisms that, due to swimming, active regulation of their buoyancy, or any other mechanism, maintain themselves in a shallow horizontal layer below the free surface of oceans or lakes. By varying the strength of the restoring force, we are able to control the thickness of the fluid slab in which the particles can move. This allows us to analyze the statistical features of the system over a wide range of conditions going from a fully 3D incompressible flow (corresponding to the case of no confinement) to the extremely confined case corresponding to a two-dimensional slice. The background 3D turbulent velocity field is evolved by means of fully resolved direct numerical simulations. Whenever some level of vertical confinement is present, the particle trajectories deviate from that of fluid tracers and the particles experience an effectively compressible velocity field. Here, we have quantified the compressibility, the preferential concentration of the particles, and the correlation dimension by changing the strength of the restoring force. The main result is that there exists a particular value of the force constant, corresponding to a mean slab depth approximately equal to a few times the Kolmogorov length scale, that maximizes the clustering of the particles

Développer l'agriculture irriguée pour consolider la sécurité alimentaire au Nord-Cameroun ?

International audienceDévelopper l'agriculture irriguée pour consolider la sécurité alimentaire au Nord-Cameroun. Au Nord-Cameroun, le déficit céréalier s'est accru ces dernières années par une combinaison de l'augmentation de la population, et de la variabilité inter annuelle des productions vivrières due aux aléas climatiques. Les mesures actuelles (régulation des prix sur le marché, promotion du stockage villageois) visant à endiguer ce déficit s'avèrent insuffisantes, et le recours aux aides alimentaires apportées par des organismes non gouvernementaux et internationaux est de plus en plus fréquent. La croissance démographique se traduit aussi par des migrations de la province de l'Extrême-Nord vers celle du Nord sur des fronts pionniers rapidement saturés et repoussés au loin. Dans ce contexte, comment envisager la couverture des besoins alimentaires de la région à court et moyen termes ? Peut-être par une meilleure valorisation des grands bassins et bas-fonds irrigables de la région encore peu exploités. En effet, à travers la maîtrise de l'eau qu'ils offrent, ils permettraient de sécuriser la production vivrière en limitant ses variations inter annuelles, et de l'augmenter par l'augmentation des superficies, l'intensification et la diversification. Cette idée n'est pas nouvelle, car Il y a quelques décennies, l'Etat a créé des structures pour la mise en valeur en régie de ces espaces (Lagdo, Maga, Yagoua...), mais les résultats n'ont jamais été à la hauteur des attentes. Mais le contexte est nouveau en ce sens que la situation alimentaire de cette région n'était pas critique comme aujourd'hui, et l'Etat se désengage de nombreuses fonctions d'encadrement de la production au profit des populations. Dans le cadre de ce processus de professionnalisation, la gestion de ces espaces aménagés par les paysans et leurs organisations est à l'ordre du jour, mais elle ne pourra pas se faire sans un accompagnement des paysans dans la prise en charge de ces nouvelles fonctions. Ce nouveau contexte pourra-t-il permettre un développement durable des aménagements hydro-agricoles et partant une amélioration de la sécurité alimentaire du Nord-Cameroun ? (Résumé d'auteur

Evaporation-Induced Crystallization of Surfactants in Sessile Multicomponent Droplets

Surfactants have been widely studied and used in controlling droplet evaporation. In this work, we observe and study the crystallization of sodium dodecyl sulfate (SDS) within an evaporating glycerol-water mixture droplet. The crystallization is induced by the preferential evaporation of water, which decreases the solubility of SDS in the mixture. As a consequence, the crystals shield the droplet surface and cease the evaporation. The universality of the evaporation characteristics for a range of droplet sizes is revealed by applying a diffusion model, extended by Raoult's law. To describe the nucleation and growth of the crystals, we employ the 2-dimensional crystallization model of Weinberg [J. Non-Cryst. Solids 1991, 134, 116]. The results of this model compare favorably to our experimental results. Our findings may inspire the community to reconsider the role of high concentration of surfactants in a multicomponent evaporation system

Association between obesity, quality of life, physical activity and health service utilization in primary care patients with osteoarthritis

OBJECTIVE: To assess the association of obesity with quality of life, health service utilization and physical activity in a large sample of primary care patients with osteoarthritis (OA). METHODS: Data were retrieved from the PraxArt project, representing a cohort of 1021 primary care patients with OA. In 978 patients, height and weight were measured and the Body Mass Index (BMI) was calculated. The AIMS2-SF was used to assess quality of life (QoL). Data about health service utilization (HSU) were retrieved by means of patients' medical files. Concomitant depression was assessed by means of the Patient Health Questionnaire (PHQ-9). Patients were grouped into normal weight, overweight and obese according to the definition of the WHO and compared by means of analysis of covariance (ANCOVA). RESULTS: Obese and overweight persons achieved significantly higher scores on the AIMS2-SF lower body scale, the symptom, the affect and the work scale, indicating an increased burden by OA. The PHQ-9 score increased significantly over the three weight-groups, indicating a positive association of BMI and depression. With increasing BMI, the number of comorbidities increased and physical activity decreased significantly. After controlling for covariates, contacts to orthopaedics and performed x-rays remained significantly higher in obese patients, but not contacts to general practitioners. CONCLUSION: The results display a strong association of QoL and BMI, resulting in increased use of the health care system. Thus, the study emphasizes the need for appropriate approaches in primary care to break the vicious circle of overweight, depression, decreasing physical inactivity and decreasing QoL

Phe161 and Arg166 variants of p-hydroxybenzoate hydroxylase Implications for NADPH recognition and structural stability

AbstractPhe161 and Arg166 of p-hydroxybenzoate hydroxylase from Pseudomonas fluorescens belong to a newly discovered sequence motif in flavoprotein hydroxylases with a putative dual function in FAD and NADPH binding [1]. To study their role in more detail, Phe161 and Arg166 were selectively changed by site-directed mutagenesis. F161A and F161G are catalytically competent enzymes having a rather poor affinity for NADPH. The catalytic properties of R166K are similar to those of the native enzyme. R166S and R166E show impaired NADPH binding and R166E has lost the ability to bind FAD. The crystal structure of substrate complexed F161A at 2.2 Å is indistinguishable from the native enzyme, except for small changes at the site of mutation. The crystal structure of substrate complexed R166S at 2.0 Å revealed that Arg166 is important for providing an intimate contact between the FAD binding domain and a long excursion of the substrate binding domain. It is proposed that this interaction is essential for structural stability and for the recognition of the pyrophosphate moiety of NADPH

Rayleigh-Taylor instability by segregation in an evaporating multi-component microdroplet

The evaporation of multi-component droplets is relevant to various applications but challenging to study due to the complex physicochemical dynamics. Recently, Li (2018) reported evaporation-triggered segregation in 1,2-hexanediol-water binary droplets. In this present work, we added 0.5 wt% silicone oil into the 1,2-hexanediol-water binary solution. This minute silicone oil concentration dramatically modifies the evaporation process as it triggers an early extraction of the 1,2-hexanediol from the mixture. Surprisingly, we observe that the segregation of 1,2-hexanediol forms plumes, rising up from the rim of the sessile droplet towards the apex during the droplet evaporation. By orientating the droplet upside down, i.e., by studying a pendant droplet, the absence of the plumes indicates that the flow structure is induced by buoyancy, which drives a Rayleigh-Taylor instability (i.e., driven by density differences & gravitational acceleration). From micro-PIV measurement, we further prove that the segregation of the non-volatile component (1,2-hexanediol) hinders the evaporation near the contact line, which leads to a suppression of the Marangoni flow in this region. Hence, on long time scales, gravitational effects play the dominant role in the flow structure, rather than Marangoni flows. We compare the measurement of the evaporation rate with the diffusion model of Popov (2005), coupled with Raoult's law and the activity coefficient. This comparison indeed confirms that the silicone-oil-triggered segregation of the non-volatile 1,2-hexanediol significantly delays the evaporation. With an extended diffusion model, in which the influence of the segregation has been implemented, the evaporation can be well described

Rotational precision MEMS-based clamping mechanism for stable fixation of elastic mechanisms

Conventional TEM sample manipulators often lack the crucial stability of 0.1 nm/min. A MEMS manipulator attached directly to the TEM pole would greatly increase both thermal and dynamic stability. However a stable E-beam requires no interference of electric or magnetic fields. Therefore the manipulator should be stably fixed without power. To this end a mechanical clamp is presented which clamps one of the actuators of the TEM sample manipulator (Figure 2). The clamp incorporates a relatively large clamp force of 0.5 mN with respect to the device area and is able to maintain the clamp force without external power. In previous work [1] a theoretical basis has been presented of an earlier clamp version. In this paper a rotational clamp which has been made and tested is presented. This clamp design is part of a research project for a 6 Degree of Freedom MEMS TEM sample manipulator. \u