Droplets move over viscoelastic substrates by surfing a ridge

Liquid drops on soft solids generate strong deformations below the contact line, resulting from a balance of capillary and elastic forces. The movement of these drops may cause strong, potentially singular dissipation in the soft solid. Here we show that a drop on a soft substrate moves by surfing a ridge: the initially flat solid surface is deformed into a sharp ridge whose orientation angle depends on the contact line velocity. We measure this angle for water on a silicone gel and develop a theory based on the substrate rheology. We quantitatively recover the dynamic contact angle and provide a mechanism for stick-slip motion when a drop is forced strongly: the contact line depins and slides down the wetting ridge, forming a new one after a transient. We anticipate that our theory will have implications in problems such as self-organization of cell tissues or the design of capillarity-based microrheometers.Comment: 9 pages, 5 figure

A compact & efficient ReBCO actuator

This study evaluates the performance of a superconducting linear-actuator demonstrator that is based on a ReBCO layer-wound solenoid. The solenoid geometry was chosen in view of ease of manufacturing and scalability. Compared to a similarly-sized copper actuator with the same power consumption, the demonstrator is designed to generate a force that is 2 orders of magnitude greater. It comprises a ReBCO solenoid stator and a concentric permanent magnet mover within in a mug-sized cylindrical envelope. When operated in boiling liquid nitrogen, a steady-state force of 300 N is targeted with a coil-level power consumption in the order of 1 mW.High-tech industry and cryogenic optical components require precise and efficient linear actuators. In terms of efficiency and heat load, power-dense HTS-based superconducting actuators perform at least an order of magnitude better than conventional ones, due to the lack of ohmic dissipation. Applications like cryogenic mirrors and detectors, where thermal load needs to be kept at a minimum, may benefit greatly from this technology. <br/

Spreading on viscoelastic solids: Are contact angles selected by Neumann's law?

The spreading of liquid drops on soft substrates is extremely slow, owing to strong viscoelastic dissipation inside the solid. A detailed understanding of the spreading dynamics has remained elusive, partly owing to the difficulty in quantifying the strong viscoelastic deformations below the contact line that determine the shape of moving wetting ridges. Here we present direct experimental visualisations of the dynamic wetting ridge using shadowgraphic imaging, complemented with measurements of the liquid contact angle. It is observed that the wetting ridge exhibits a rotation that follows exactly the dynamic liquid contact angle - as was previously hypothesized [Karpitschka et al., Nat. Commun., 2015, 6, 7891]. This experimentally proves that, despite the contact line motion, the wetting ridge is still governed by Neumann's law. Furthermore, our experiments suggest that moving contact lines lead to a variable surface tension of the substrate. We therefore set up a new theory that incorporates the influence of surface strain, for the first time including the so-called Shuttleworth effect into the dynamical theory for soft wetting. It includes a detailed analysis of the boundary conditions at the contact line, complemented by a dissipation analysis, which shows, again, the validity of Neumann's balance

The bifurcation diagram of drops in a sphere/plane geometry: influence of contact angle hysteresis

We study liquid drops that are present in a generic geometry, namely the gap in between a sphere and a plane. For the ideal system without contact angle hysteresis, the drop position is solely dependent on the contact angle, drop volume, and sphere/ plane separation distance. Performing a geometric analysis and Surface Evolver calculations, a continuous and fully reversible transition between axisymmetric non-spherical shapes and non-axisymmetric spherical shapes is predicted. We also study these transitions experimentally, varying the contact angle using electrowetting. Then, pinning forces drastically alter the pitchfork bifurcation as the unstable branch gets stabilized, and introduce a history-dependence in the system. As a consequence, the outward movement of drops following pinning can be either continuous or discontinuous, depending on the minimum contact angle that is attained

Soft wetting: Models based on energy dissipation or on force balance are equivalent

In Newtonian mechanics, an overdamped system at steady state is governed by a local balance of mechanical stress but also obeys a global balance between injected and dissipated energy. In the classical literature of purely viscous drop spreading, apparent differences in “dissipation” and “force” approaches have led to unnecessary debates, which ultimately could be traced back to different levels of mathematical approximation (1). In the context of wetting on a soft solid, Zhao et al. (2) interpret their experiments by a model based on viscoelastic dissipation inside the substrate. It is claimed that this global dissipation model is fundamentally different from the local mechanical model presented by Karpitschka et al. (3). The purpose of this letter is to demonstrate that (i) the models in refs. 2 and 3 are in fact strictly equivalent … ↵1To whom correspondence should be addressed. Email: stefan.karpitschka{at}ds.mpg.de

Huidklachten door cosmetische producten : Trendrapportage 2012 - 2013

Cosmetica zijn in principe veilig, maar kunnen soms huidklachten veroorzaken, zoals roodheid en jeuk. Het RIVM beheert een systeem waarin deze klachten en andere overgevoeligheidsreacties na gebruik van cosmetica kunnen worden geregistreerd (CESES, Consumer Exposure Skin Effects and Surveillance). Net als in voorgaande jaren zijn dergelijke klachten in 2012-2013 vooral gemeld na het gebruik van haarproducten, huidverzorgingsproducten en make-up. De klachten doen zich vooral voor bij producten die bedoeld zijn voor gebruik op of rond de ogen. Daarnaast hebben relatief veel kappers contacteczeem op de handen na het gebruik van haarproducten op hun werk. Isothiazolinonen, gebruikt als conserveringsmiddel in cosmetica, en geurstoffen blijven de ingrediënten die het vaakst allergische reacties veroorzaken. Extra aandacht voor isothiazolinonen, zowel wat betreft regelgeving als onderzoek, is van belang, omdat deze stoffen ook in andere consumenten- en industriële producten gebruikt worden. Dit maakt het voor consumenten die overgevoelig zijn voor isothiazolinonen lastig om deze stoffen te vermijden. Voor kappers blijven ammoniumpersulfaat en PPD de belangrijkste veroorzakers van contacteczeem. Net als in voorgaande jaren zijn in 2012 en 2013 allergische reacties gemeld op het UV-filter octocryleen, dat in zonnebrandcrème zit, en op co-/crosspolymeren, die ook in bepaalde crèmes gebruikt worden. CESES wordt gebruikt om na te gaan of Europese wetgeving en handhaving de consument voldoende beschermt. Ook kunnen risico's voor werknemers worden geïdentificeerd. Consumenten kunnen zelf hun klacht melden via de website www.cosmeticaklachten.nl. Daarnaast registreren deelnemende dermatologen huidklachten van patiënten waarbij cosmetica de mogelijke oorzaak zijn. Bij deze patiënten wordt vervolgens een allergieonderzoek uitgevoerd om vast te stellen welk(e) productingrediënt(en) de klacht veroorzaakt.Cosmetics are in principle safe to use. In some cases however, cosmetic products may lead to undesirable reactions, such as itching and erythema. RIVM has set up a monitoring system in which undesirable reactions as well as other allergic reactions caused by cosmetics can be registered (CESES, Consumer Exposure Skin Effects and Surveillance). As in previous years, such reactions are mainly reported after the use of hair products, skin products and make-up, including primarily those products intended to use on or around the eyes. In addition, relatively many hairdressers reported contact dermatitis located on the hands after the use of hair products at their work. Isothiazolinones, a preservative in cosmetics, and fragrances remain the cosmetic ingredients relatively most responsible for allergic reactions. More attention to isothiazolinones is especially important, because these ingredients are also widely used in other consumer and industrial products. As a result, it is difficult for consumers, who are allergic to isothiazolinones, to avoid these ingredients. Ammonium persulfates and PPD remain the most important causative agents for contact dermatitis in hairdressers. As in previous years, allergic reactions to the UV filter octocrylene, which is used in sunscreens, and co/cross polymers, which are used in several creams, were reported. The goal of CESES is to monitor undesirable reactions attributable to cosmetics and cosmetic ingredients to assess whether current EU legislation on cosmetics provides adequate consumer protection. Also, risks for workers can be identified. Consumers can report allergic reactions on the website www.cosmeticaklachten.nl. In addition, participating dermatologists report cases of contact dermatitis to the system when cosmetics are expected to be the cause. Dermatologists also carried out patch tests and, where necessary, tests with specific batch ingredients of the associated cosmetic product.NVW

Droplets move over viscoelastic substrates by surfing a ridge

\u3cp\u3eLiquid drops on soft solids generate strong deformations below the contact line, resulting from a balance of capillary and elastic forces. The movement of these drops may cause strong, potentially singular dissipation in the soft solid. Here we show that a drop on a soft substrate moves by surfing a ridge: the initially flat solid surface is deformed into a sharp ridge whose orientation angle depends on the contact line velocity. We measure this angle for water on a silicone gel and develop a theory based on the substrate rheology. We quantitatively recover the dynamic contact angle and provide a mechanism for stick-slip motion when a drop is forced strongly: the contact line depins and slides down the wetting ridge, forming a new one after a transient. We anticipate that our theory will have implications in problems such as self-organization of cell tissues or the design of capillarity-based microrheometers.\u3c/p\u3

Huidklachten door cosmetische producten : Jaarrapport CESES

Ruim 1.250 meldingen van klachten bij gebruik van cosmetica Bij het meldpunt Cosmeticaklachten.nl en via deelnemende huisartsen en dermatologen zijn in het eerste jaar van het bestaan van het project Consumer Exposure Skin Effects and Surveillance (CESES) ruim 1.250 meldingen binnengekomen van huidklachten die mogelijk door cosmetische producten zijn veroorzaakt. De meest genoemde symptomen zijn roodheid en jeuk. De klachten komen vooral voor op of rondom de oogleden en in het gezicht. De meest genoemde productcategorieën zijn huidverzorgingsproducten, haarverzorgingsproducten en make-up. Het gaat voornamelijk om dag- en nachtcrèmes, shampoos en oogmake-up (mascara, eyeliner, oogpotlood en oogmake-up remover). Daarnaast werden relatief veel klachten over zonnecosmetica gemeld. De klachten worden meestal door vrouwen gemeld (80-90% van de meldingen). Van de mensen die hun klacht via de website hebben gemeld is 37% met de klacht naar de huisarts geweest en voor 13% was een bezoek aan een dermatoloog noodzakelijk. Slechts 17% van de respondenten gaat met het product terug naar de winkel en 10% neemt contact op met de fabrikant. Opvallend is dat slechts 30% van de respondenten bekend is met het houdbaarheidssymbool of datum van houdbaarheid op cosmetische producten. Er worden heel weinig klachten bij baby's en jonge kinderen gemeld. Ook over geuren komen weinig klachten binnen. Dit is opvallend omdat geurstoffen erkende contactallergenen in consumentenproducten zijn. Op basis van de ernst van de meldingen uit de publieke route heeft het RIVM drie keer een attendering (early warning) uitgestuurd naar de nVWA met betrekking tot een specifiek product. Naar aanleiding van deze attenderingen heeft de nVWA contact opgenomen met de fabrikant. Op basis van de reactie van de fabrikant heeft de nVWA geconcludeerd dat er in het geval van deze drie early warnings geen sprake is van onveilige producten.Over 1,250 reports of adverse effects after the use of cosmetic products. Since the start of the CESES (Consumer Exposure Skin Effects and Surveillance) project, more than 1,250 reports of adverse reactions to cosmetic products have been received via the website www.cosmeticaklachten.nl and from participating family doctors and dermatologists. The most commonly reported adverse effects are erythema and itching, predominantly those localized around the eye(lids) and on the face. The most frequently mentioned product categories are skin care products, hair care products and make-up. In particular, there have been many reports of day- and night creams, shampoo and eye make-up (mascara, eyeliner, eye pencil and eye make-up remover) causing an adverse effect. A relatively high number of complaints about sun care products have also been received. Women reported 80-90% of the adverse effects. Of the consumers who reported an adverse effect on the website, 37% had visited a family doctor and 13% had consulted a dermatologist. Only 17% of the consumers had returned to the store where they had bought the product, and only 10% had contacted the manufacturer. Worthy of note is that only 30% of the respondents were familiar with the shelf life symbol or the note is that only 30% of the respondents were familiar with the shelf life symbol or the period of time after opening that the cosmetic product may be used without any harm to the consumer. Hardly any adverse effects were reported for babies and young children. There were also very few reports of adverse effects associated with perfumes, which is remarkable because fragrance allergens are considered to be the most important allergenic substances present in consumer products. Based on the severity of reports received to date via the website, RIVM has on three different occasions sent an alert to the nVWA (Food and Consumer Product Safety Authority) concerning a specific cosmetic product. After contacting the manufacturer, the nVWA has concluded that these cosmetic products do not pose a risk to the consumer.VW