Avalanche of particles in evaporating coffee drops

The pioneering work of Deegan et al. [Nature 389, (1997)] showed how a drying sessile droplet suspension of particles presents a maximum evaporating flux at its contact line which drags liquid and particles creating the well known coffee stain ring. In this Fluid Dynamics Video, measurements using micro Particle Image Velocimetry and Particle Tracking clearly show an avalanche of particles being dragged in the last moments, for vanishing contact angles and droplet height. This explains the different characteristic packing of the particles in the layers of the ring: the outer one resembles a crystalline array, while the inner one looks more like a jammed granular fluid. Using the basic hydrodynamic model used by Deegan et al. [Phys. Rev. E 62, (2000)] it will be shown how the liquid radial velocity diverges as the droplet life comes to an end, yielding a good comparison with the experimental data.Comment: This entry contains a Fluid Dynamics Video candidate for the Gallery of Fluid Motion 2011 and a brief article with informatio

Whipping Instabilities in Electrified Liquid Jets

A liquid jet may develop different types of instabilities, like the so-called Rayleigh-Plateau instability, which breaks the jet into droplets. However, another type of instabilities may appear when we electrify a liquid jet and induce some charge at his surface. Among them, the most common is the so-called Whipping Instability, which is characterized by violent and fast lashes of the jet. In the submitted fluid dynamic video(see http://hdl.handle.net/1813/11422), we will show an unstable charged glycerine jet in a dielectric liquid bath, which permits an enhanced visualization of the instability. For this reason, it is probably the first time that these phenomena are visualized with enough clarity to analyze features as the effect of the feeding liquid flow rate through the jet or as the surprising spontaneous stabilization at some critical distance to the ground electrode.Comment: 3 pages, no figures, links to videos, Submission to the 26th Gallery of Fluid Motion (2009

Role of liquid driving on the clogging of constricted particle suspensions

Forcing dense suspensions of non-cohesive particles through constrictions might either result in a continuous flow, an intermittent one, or indefinite interruption of flow, i.e., a clog. While one of the most important (and obvious) controlling parameters in such a system is the neck-to-particle size ratio, the role of the liquid driving method is not so obvious. On the one hand, wide-spread volume-controlled systems result in pressure and local liquid velocity increases upon eventual clogs. On the other hand, pressure-controlled systems result in a decrease of the flow through the constriction when a clog is developed. The root of the question therefore lies on the role of interparticle liquid flow and hydrodynamic forces on both the formation and stability of an arch blocking the particle transport through a constriction. In this work, we experimentally analyse a suspension of non-cohesive particles in channels undergoing intermittent regimes, in which they are most sensitive to parametric changes. By exploring the statistical distribution of arrest times and of discharged particles, we surprisingly find that the transport of non-cohesive suspensions through constrictions actually follows a "slower is faster" principle under certain conditions.Comment: 9 pages, 5 figures, 2 table

Single-Camera 3D PTV Methods for Evaporation-Driven Liquid Flows in Sessile Droplets

The experimental characterization of liquid flows in sessile evaporating droplets is an important task for the fundamental understanding of the complex phenomena occurring in these apparently simple systems. The liquid flow induced by the droplet evaporation has a strong three-dimensional character and conventional visualization methods are typically difficult to apply. A more effective approach is to look inside the droplets from the substrate where the droplet lies and use single-camera 3D particle tracking velocimetry (PTV) methods to reconstruct the whole flow field. This paper discusses the implementation of an experimental setup for the quantitative characterization of the flow inside sessile evaporating droplets based on two single-camera 3D PTV methods: the Astigmatic Particle Tracking Velocimetry (APTV) and the General Defocusing Particle Tracking (GDPT). Exemplary results on different types of sessile evaporating droplets are reported and discussed. The presented approach is easy to implement, does not require special or costly equipment, and has the potential to become a standard tool for this type of experiments.</p