Abstract
Microscopic imaging as well as the particle image velocimetry (PIV) are carried out to evaluate the concentration, velocity and vorticity fields near the contact line of the nano-particles-laden evaporating sessile droplets. After the onset of the linear thermocapillary instabilities due to the Marangoni perturbations, the non-linear state sets in and the micro-scale jet-like vortex structures are ejected from the contact line towards the center of the droplet. Afterwards, the jet-like vortical structures expand in the spanwise directions and start to interact with the neighbouring structures. Two types of the inverse cascade mechanisms are found to occur. In the first kind, the vortices of the similar length scale merge and continuously produce larger vortices and corresponding wavelength growth. The second inverse cascade mechanism takes place due to the entrainment of the smaller vortices into the larger structures. Both inverse cascade processes are identified as the continuous feeding of the kinetic energy from the smaller scales to the larger scales. For individual micro-jets the velocity field characterizes the jet-like vortex structures ejected from the contact line towards the droplet center opposing the bulk flow from the center towards the contact line. In addition, the vorticity field overlaid by the velocity streamlines identify the sense of rotation of the low pressure zones on either side of the micro-jet as well as the high pressure stagnation point at the tip.
Highlights
Present communication reports the observation of the co-occurrence of two remarkable phenomena which have fascinated many researchers working in the fields of the physical sciences and thermofluids
The remarkable micro scale vortex dynamics phenomena have been repeatedly observed for all the 47 cases studied in the present investigation
The coffee ring effect is attributed to the darker region of the dried pattern remaining near the contact line of the evaporating sessile droplet
Summary
Present communication reports the observation of the co-occurrence of two remarkable phenomena which have fascinated many researchers working in the fields of the physical sciences and thermofluids. The first phenomenon is the onset and growth of the thermocapillary instabilities near the contact line of the nano-particle-laden evaporating sessile droplets, and the second mechanism is the concept of the inverse energy cascade of the associated vortical structures. From a fundamental point of view, parameters such as the onset of the thermocapillary instabilities arising during the heat/mass transfer[10,11], the shape of the colloidal particles[12], substrate heat transfer characteristics[10], ambient temperature and relative humidity[13], substrate hydrophobicity[14], and the microscopic surface roughness[15] may either result in the formation of a coffee-ring or a homogeneous deposition by altering the internal fluid flow of the sessile droplet. The vortical structures of different scales, or the so-called eddies, seem to co-exist and interact through the momentum transfer[20] Such eddies are predominantly observed in the nature and continuously exchange kinetic energy within the variety of fluid flows. The occurrence of the inverse energy cascade associated with the micro-scale vortex structures is described
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