Denis Richard

Bio: Denis Richard is an academic researcher from Collège de France. The author has contributed to research in topics: Drop (liquid) & Wetting. The author has an hindex of 7, co-authored 10 publications receiving 2349 citations.

Contact time of a bouncing drop

Abstract: When a liquid drop lands on a solid surface without wetting it, it bounces with remarkable elasticity. Here we measure how long the drop remains in contact with the solid during the shock, a problem that was considered by Hertz for a bouncing ball. Our findings could help to quantify the efficiency of water-repellent surfaces (super-hydrophobic solids) and to improve water-cooling of hot solids, which is limited by the rebounding of drops as well as by temperature effects.

Maximal deformation of an impacting drop

Abstract: We first study the impact of a liquid drop of low viscosity on a super-hydrophobic surface. Denoting the drop size and speed as are the liquid density and surface tension). This law is also observed to hold on partially wettable surfaces, provided that liquids of low viscosity (such as water) are used. The law is interpreted as resulting from the effective acceleration experienced by the drop during its impact. Viscous drops are also analysed, allowing us to propose a criterion for predicting if the spreading is limited by capillarity, or by viscosity.

Bouncing water drops

Abstract: When a liquid drop impacts a solid, it spreads (with possibly beautiful fingering patterns) up to the point when kinetic energy is dissipated by viscosity. Then, it can retract (if the solid is partially wetted by the liquid), or not. A very different behaviour can be observed on highly hydrophobous solids. On such solids, the contact angle is close to 180°, so that the kinetic energy of the impinging drop can be transferred to surface energy, without spreading. Thus, the drop can fully bounce. However, the liquid nature of this kind of spring imposes a limit for the restitution coefficient, which is due to the fact that the drop, after the lift-off, oscillates.

Water spring: A model for bouncing drops

Abstract: It has been shown that a water drop can bounce persistently, when thrown on a super-hydrophobic substrate. We present here scaling arguments which allow us to predict the maximal deformation and the contact time of the drop. This approach is completed by a model describing the flow inside the drop, and by original experimental data.

Viscous drops rolling on a tilted non-wettable solid

Abstract: A viscous liquid drop sliding down an inclined solid that it partially wets runs all the faster since it is large. Here we examine what happens in the limit of very high contact angles, on a so-called super-hydrophobic surface. It is shown that a droplet rolls instead of sliding, which leads to a surprising law for the velocity as a function of the drop radius: the smaller the droplet, the larger the running velocity. A recent model of Mahadevan and Pomeau allows us to propose an explanation for this paradoxical behaviour.

Wetting and Roughness

Abstract: We discuss in this review how the roughness of a solid impacts its wettability. We see in particular that both the apparent contact angle and the contact angle hysteresis can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well-controlled micro- or nanotextures on a solid, these effects are being exploited to induce novel wetting properties, such as spontaneous filmification, superhydrophobicity, superoleophobicity, and interfacial slip, that could not be achieved without roughness.

Self-cleaning surfaces - virtual realities

Abstract: In the 19th century, Oscar Wilde stated “We live, I regret to say, in an age of surfaces”. Today, we do so even more, and we do not regret it: key advances in the understanding and fabrication of surfaces with controlled wetting properties are about to make the dream of a contamination-free (or 'no-clean') surface come true. Two routes to self-cleaning are emerging, which work by the removal of dirt by either film or droplet flow. Although a detailed understanding of the mechanisms underlying the behaviour of liquids on such surfaces is still a basic research topic, the first commercial products in the household-commodity sector and for applications in biotechnology are coming within reach of the marketplace. This progress report describes the current status of understanding of the underlying mechanisms, the concepts for making such surfaces, and some of their first applications.

Drop Impact Dynamics: Splashing, Spreading, Receding, Bouncing ...

Abstract: The review deals with drop impacts on thin liquid layers and dry surfaces. The impacts resulting in crown formation are referred to as splashing. Crowns and their propagation are discussed in detail, as well as some additional kindred, albeit nonsplashing, phenomena like drop spreading and deposition, receding (recoil), jetting, fingering, and rebound. The review begins with an explanation of various practical motivations feeding the interest in the fascinating phenomena of drop impact, and the above-mentioned topics are then considered in their experimental, theoretical, and computational aspects.

Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating

Abstract: Coating is an essential step in adjusting the surface properties of materials. Superhydrophobic coatings with contact angles greater than 150° and roll-off angles below 10° for water have been developed, based on low-energy surfaces and roughness on the nano- and micrometer scales. However, these surfaces are still wetted by organic liquids such as surfactant-based solutions, alcohols, or alkanes. Coatings that are simultaneously superhydrophobic and superoleophobic are rare. We designed an easily fabricated, transparent, and oil-rebounding superamphiphobic coating. A porous deposit of candle soot was coated with a 25-nanometer-thick silica shell. The black coating became transparent after calcination at 600°C. After silanization, the coating was superamphiphobic and remained so even after its top layer was damaged by sand impingement.

Transformation of a simple plastic into a superhydrophobic surface.

Abstract: Superhydrophobic surfaces are generally made by controlling the surface chemistry and surface roughness of various expensive materials, which are then applied by means of complex time-consuming processes. We describe a simple and inexpensive method for forming a superhydrophobic coating using polypropylene (a simple polymer) and a suitable selection of solvents and temperature to control the surface roughness. The resulting gel-like porous coating has a water contact angle of 160 degrees. The method can be applied to a variety of surfaces as long as the solvent mixture does not dissolve the underlying material.