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.

We derive analytical expressions for the three-dimensional (3D) acoustophoretic motion of spherical microparticles in rectangular microchannels. The motion is generated by the acoustic radiation force and the acoustic streaming-induced drag force. In contrast to the classical theory of Rayleigh streaming in shallow, infinite, parallel-plate channels, our theory does include the effect of the microchannel side walls. The resulting predictions agree well with numerics and experimental measurements of the acoustophoretic motion of polystyrene spheres with nominal diameters of 0.537 µm and 5.33 µm. The 3D particle motion was recorded using astigmatism particle tracking velocimetry under controlled thermal and acoustic conditions in a long, straight, rectangular microchannel actuated in one of its transverse standing ultrasound-wave resonance modes with one or two half-wavelengths. The acoustic energy density is calibrated in situ based on measurements of the radiation dominated motion of large 5-µm-diam particles, allowing for quantitative comparison between theoretical predictions and measurements of the streaming induced motion of small 0.5-µm-diam particles.

A drop of water deposited on a cold plate freezes into an ice drop with a pointy tip. While this phenomenon clearly finds its origin in the expansion of water upon freezing, a quantitative description of the tip singularity has remained elusive. Here we demonstrate how the geometry of the freezing front, determined by heat transfer considerations, is crucial for the tip formation. We perform systematic measurements of the angles of the conical tip, and reveal the dynamics of the solidification front in a Hele-Shaw geometry. It is found that the cone angle is independent of substrate temperature and wetting angle, suggesting a universal, self-similar mechanism that does not depend on the rate of solidification. We propose a model for the freezing front and derive resulting tip angles analytically, in good agreement with the experiments.

An evaporating droplet is a dynamic system in which flow is spontaneously generated to minimize the surface energy, dragging particles to the borders and ultimately resulting in the so-called "coffee-stain effect". The situation becomes more complex at the droplet's surface, where surface tension gradients of different natures can compete with each other yielding different scenarios. With careful experiments and with the aid of 3D particle tracking techniques, we are able to show that different types of surfactants turn the droplet's surface either rigid or elastic, which alters the evaporating fluid flow, either enhancing the classical coffee-stain effect or leading to a total flow inversion. Our measurements lead to unprecedented and detailed measurements of the surface tension difference along the evaporating droplet's surface with good temporal and spatial resolution.

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 flow of a charged-stabilized suspension through a single constricted channel is studied experimentally by tracking the particles individually. Surprisingly, the behavior is found to be qualitatively similar to that of inertial dry granular systems: For small values of the neck-to-particle size ratio (D/d<3), clogs form randomly as arches of the particle span the constriction. The statistics of the clogging events are Poissonian as reported for granular systems and agree for moderate particle volume fraction (ϕ≈20%) with a simple stochastic model for the number of particles at the neck. For larger neck sizes (D/d>3), even at the largest ϕ(≈60%) achievable in the experiments, an uninterrupted particle flow is observed, which resembles that of an hourglass. This particularly small value of D/d(≃3) at the transition to a practically uninterrupted flow is attributed to the low effective friction between the particles, achieved by the particle's functionalization and lubrication.

Evaporating salty droplets are ubiquitous in nature, in our home and in the laboratory. Interestingly, the transport processes in such apparently simple systems differ strongly from evaporating "freshwater" droplets since convection is partly inverted due to Marangoni stresses. Such an effect has crucial consequences to the salt crystallization process and to the deposits left behind. In this work we show unprecedented measurements that, not only confirm clearly the patterns of the flow inversion, but also elucidate their impact on the distribution of non-volatile solutes. Contrary to what has been often reported in the literature, such a flow reversal does not prevent the formation of ring-shaped stains: particles accumulate at the contact line driven solely by the interfacial flow. We can therefore conclude that the classical "coffee-stain effect" is not the only mechanism that can generate ring-shaped stains in evaporating droplets.