4 research outputs found
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Velocity measurement during evaporation of seeded, sessile drops on heated surfaces
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Evaporation of sessile drops has been studied extensively in an attempt to understand the effect of wetting on the evaporation process. Recently interest has also increased in the deposition of particles from such drops, with evaporative mass flux being deemed to be responsible for ring-like deposits and Marangoni convection counteracting this mass flux explaining more uniform deposition patterns. Understanding of such deposition processes is important in ink-jet printing and other micro-scale deposition technologies, where the nature of deposition can have a dramatic effect on the quality or effectiveness of the finished product. In most cases where deposition from evaporating drops has been studied, velocity information is inferred from the final deposition pattern or from mathematical modeling based on simplified models of the physics of the evaporation process. In this study we have directly measured the flow velocities in the base of sessile drops,
using micro-PIV, viewing the drop from below, through the cover slide. The images obtained have also enabled us to observe the formation of holes in the liquid film during the latter stages of evaporation
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Flow measurement using micro-PIV and related temperature distributions within evaporating sessile drops of self-rewetting mixtures of 1-pentanol and water
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Recently interest has arisen in the use of so-called self-rewetting mixtures for micro-scale heat
transfer systems. Such fluids, in which the surface tension can increase with increasing temperature, are
expected to offer superior evaporative cooling performance by extending the region of operation before dryout
of the heated surface sets in. Whilst improved performance has been shown in some practical situations
using these fluids, it is not entirely clear as to the mechanism of such improvements.
We have studied the flow within evaporating sessile drops of 1-pentanol-water mixtures using micro-PIV
and have observed three stages in the evaporation process. During the first stage there appears to be a single
toroidal vortex with flow inwards along the base of the drop. The vortex only occupies the central region of
the drop and appears to pulsate, reducing in size during evaporation. This is followed by a second transition
stage to a third stage in which the flow is directed radially outward, as observed by us for pure water droplet
evaporation and in the latter stages of ethanol/water drop evaporation. Temperature measurements, using IR
thermography suggest that the initial stage of evaporation may be controlled by thermal Marangoni effects as
opposed to the concentration driven Marangoni flows postulated for ethanol-water mixtures
Dynamics and universal scaling law in geometrically-controlled sessile drop evaporation
The evaporation of a liquid drop on a solid substrate is a remarkably common phenomenon. Yet, the complexity of the underlying mechanisms has constrained previous studies to sphericallysymmetric configurations. Here we investigate well-defined, non-spherical evaporating drops of pure liquids and binary mixtures. We deduce a universal scaling law for the evaporation rate valid for any shape and demonstrate that more curved regions lead to preferential localized depositions in particle-laden drops. Furthermore, geometry induces well-defined flow structures within the drop that change according to the driving mechanism. In the case of binary mixtures, geometry dictates the spatial segregation of the more volatile component as it is depleted. Our results suggest that the drop geometry can be exploited to prescribe the particle deposition and evaporative dynamics of pure drops and the mixing characteristics of multicomponent drops, which may be of interest to a wide range of industrial and scientific applications