A colloidal dispersion droplet evaporating from a surface, such as a drying coffee drop, leaves a distinct ring-shaped stain. Although this mechanism is frequently used for particle self-assembly, the conditions for crystallization have remained unclear. Our experiments with monodisperse colloidal particles reveal a structural transition in the stain, from ordered crystals to disordered packings. We show that this sharp transition originates from a temporal singularity of the flow velocity inside the evaporating droplet at the end of its life. When the deposition speed is low, particles have time to arrange by Brownian motion, while at the end, high-speed particles are jammed into a disordered phase.

Evaporation of water droplets on a superhydrophobic substrate, on which the contact line is pinned, is investigated. While previous studies mainly focused on droplets with contact angles smaller than 90 • , here we analyze almost the full range of possible contact angles (10 • -150 • ). The greater contact angles and pinned contact lines can be achieved by the use of superhydrophobic Carbon Nanofiber substrates. The time-evolutions of the contact angle and the droplet mass are examined. The experimental data is in good quantitative agreement with the model presented by Popov (Physical Review E 71, 2005), demonstrating that the evaporation process is quasi-static, diffusion-driven, and that thermal effects play no role. Furthermore, we show that the experimental data for the evolution of both the contact angle and the droplet mass can be collapsed onto one respective universal curve for all droplet sizes and initial contact angles.

Keldermann RH, Nash MP, Gelderblom H, Wang VY, Panfilov AV. Electromechanical wavebreak in a model of the human left ventricle. Am J Physiol Heart Circ Physiol 299: H134 -H143, 2010. First published April 16, 2010; doi:10.1152/ajpheart.00862.2009In the present report, we introduce an integrative three-dimensional electromechanical model of the left ventricle of the human heart. Electrical activity is represented by the ionic TP06 model for human cardiac cells, and mechanical activity is represented by the NiedererHunter-Smith active contractile tension model and the exponential Guccione passive elasticity model. These models were embedded into an anatomic model of the left ventricle that contains a detailed description of cardiac geometry and the fiber orientation field. We demonstrated that fiber shortening and wall thickening during normal excitation were qualitatively similar to experimental recordings. We used this model to study the effect of mechanoelectrical feedback via stretch-activated channels on the stability of reentrant wave excitation. We found that mechanoelectrical feedback can induce the deterioration of an otherwise stable spiral wave into turbulent wave patterns similar to that of ventricular fibrillation. We identified the mechanisms of this transition and studied the three-dimensional organization of this mechanically induced ventricular fibrillation. ventricular fibrillation; computer simulations; mechanics; electrophysiology; mechanoelectrical feedback; stretch-activated channels MECHANICAL ACTIVITY of the heart is initiated by electrical waves of excitation that propagate through the heart and initiate cardiac contraction. Abnormal excitation of the heart may result in cardiac arrhythmias and the loss of mechanical pump function, leading to sudden cardiac death. Sudden cardiac death caused by cardiac arrhythmias is the most common cause of death in the industrialized world, and, in most cases, this is due to ventricular fibrillation (VF) (73). It has been shown in clinical and experimental studies (11,17,26,41,46,62,68,70) that VF occurs as a result of the onset of turbulent electrical activation patterns of the heart that are underpinned by multiple reentrant sources of excitation. Mechanisms behind the onset of reentrant sources in the heart and processes resulting in breakup of these sources into complex turbulent activation patterns are of great interest, e.g., in the design of therapeutic strategies to prevent or treat cardiac arrhythmias.One of the important factors that affects electrical excitation of the heart is mechanoelectrical feedback. It has been shown that mechanical deformation alters the electrical properties of myocytes via stretch-activated channels (59), which can change the shape of the action potential in response to stretch (34, 37). Mechanoelectric feedback has been studied in the clinical community for well over a century (for reviews, see Refs. 34,37,38) and may have both proarrhythmic and antiarrhythmic consequences. However, the mechanisms underlying these ph...

We show how the deposition of laser energy induces propulsion and strong deformation of an absorbing liquid body. Combining high speed with stroboscopic imaging, we observe that a millimeter-sized dyed water drop hit by a millijoule nanosecond laser pulse propels forward at several meters per second and deforms until it eventually fragments. The drop motion results from the recoil momentum imparted at the drop surface by water vaporization. We measure the propulsion speed and the time-deformation law of the drop, complemented by boundary-integral simulations. The drop propulsion and shaping are explained in terms of the laser-pulse energy, the drop size, and the liquid properties. These findings are, for instance, crucial for the generation of extreme ultraviolet light in nanolithography machines.

Evaporation-driven particle self-assembly can be used to generate three-dimensional microstructures. We present a unique method to create colloidal microstructures in which we can control the amount of particles and their packing fraction. To this end, we evaporate colloidal dispersion droplets on a special type of superhydrophobic microstructured surface, on which the droplet remains in Cassie-Baxter state during the entire evaporative process. The remainders of the droplet consist of a massive spherical cluster of the microspheres, with diameters ranging from a few tens up to several hundreds of microns. We present scaling arguments to show how the final particle packing fraction of these balls depends on the dynamics of the droplet evaporation, particle size, and number of particles in the system. superhydrophobicity | microparticle deposition E vaporation-driven particle self-assembly is an ideal mechanism for constructing micro-and nanostructures at scales where direct manipulation is impossible. For example, in colloidal dispersion droplets with pinned contact lines, evaporation gives rise to the so-called coffee stain effect (1): A capillary flow drags the particles toward the contact line to form a ring-shaped stain. Such a flow not only aggregates the particles, but is also able to organize them in crystalline phases (2-5). Similar mechanisms such as the convective assembly (6, 7) are currently successfully used to produce two-dimensional colloidal crystal films. To obtain three-dimensional clusters of microparticles, colloidal dispersion droplets can be dried suspended in emulsions (8-10), in spray dryers (11, 12), or kept in Leidenfrost levitation (13). The main drawback of these three-dimensional assembly techniques, however, is the lack of control on both the amount of particles and the particle arrangement in the remaining structures.In this work, we devise a unique, controlled way of generating on-demand self-assembled spherical microstructures via droplet evaporation on a superhydrophobic surface (Fig. 1). We present scaling arguments to predict the particle arrangement in the microstructures formed, based on the dynamics of the evaporation process. To generate the microstructures, we evaporate colloidal dispersion droplets on a special type of superhydrophobic substrates. In most of the cases, a liquid Cassie-Baxter state drop evaporating on a superhydrophobic surface will eventually suffer a wetting transition into a Wenzel state, i.e., it will get impaled into the structure and loose its spherical shape (14, 15). Here, however, we use a surface that combines overhanging pillared structures (16, 17) with a hierarchical nanostructure (Fig. 2C). These surface properties impose a huge energy barrier for the wetting transition to occur, and therefore the droplet will remain almost floating over the structure in a Cassie-Baxter state during its entire life (18).A typical result can be observed in Fig. 1 (see also Movie S1: A water droplet containing 1 μm soluble polystyrene particles (initial concen...

The propulsion of a liquid indium-tin micro-droplet by nanosecond-pulse laser impact is experimentally investigated. We capture the physics of the droplet propulsion in a scaling law that accurately describes the plasma-imparted momentum transfer, enabling the optimization of the laser-droplet coupling. The subsequent deformation of the droplet is described by an analytical model that accounts for the droplet's propulsion velocity and the liquid properties. Comparing our findings to those from vaporization-accelerated mm-sized water droplets, we demonstrate that the hydrodynamic response of laser-impacted droplets is scalable and independent of the propulsion mechanism.

The evaporation of sessile drops in quiescent air is usually governed by vapour diffusion. For contact angles below $90^\circ$, the evaporative flux from the droplet tends to diverge in the vicinity of the contact line. Therefore, the description of the flow inside an evaporating drop has remained a challenge. Here, we focus on the asymptotic behaviour near the pinned contact line, by analytically solving the Stokes equations in a wedge geometry of arbitrary contact angle. The flow field is described by similarity solutions, with exponents that match the singular boundary condition due to evaporation. We demonstrate that there are three contributions to the flow in a wedge: the evaporative flux, the downward motion of the liquid-air interface and the eigenmode solution which fulfils the homogeneous boundary conditions. Below a critical contact angle of $133.4^\circ$, the evaporative flux solution will dominate, while above this angle the eigenmode solution dominates. We demonstrate that for small contact angles, the velocity field is very accurately described by the lubrication approximation. For larger contact angles, the flow separates into regions where the flow is reversing towards the drop centre.Comment: Journal of Fluid Mechanics 709 (2012

When does a drop, deposited on a cold substrate, stop spreading? Despite the practical relevance of this question, for example, in airplane icing and three-dimensional metal printing, the detailed mechanism of arrest in solidifying spreading drops has remained debated. Here we consider the spreading and arrest of hexadecane drops of constant volume on two smooth wettable substrates: copper with a high thermal conductivity and glass with a low thermal conductivity. We record the spreading radius and contact angle in time for a range of substrate temperatures. The experiments are complemented by a detailed analysis of the temperature field near the rapidly moving contact line, by means of similarity solutions of the thermohydrodynamic problem. Our combined experimental and theoretical results provide strong evidence that the spreading of solidifying drops is arrested when the liquid at the contact line reaches a critical temperature, which is determined by the effect of kinetic undercooling.