A study on the dynamic behaviors of water droplets impacting nanostructured surfaces AIP Advances 1, 042139 (2011) Impact behavior of droplets on a surface is an intriguing research topic, and its control should be very useful in diverse industrial applications. We investigated the impact behavior of water droplets on the textured and chemically treated surface of silicon and obtained the impact mode map on the parameter plane subtended by the Weber number (up to 85) and temperature (up to 320 C). The patterns comprise of micropillars (14 lm in height) in square lattice with a lattice constant of 10 and 20 lm, and the surface was further made superhydrophobic by coating with graphene nanosheets. Six distinct impact modes are identified. It was found that the impact mode map can be dramatically altered by modifying the texture and chemistry of the surface, and the observations are well explained with regard to heat transfer, vapor/bubble generation and vapor flow beneath the droplet. Instability in the droplet arising from the mismatch between vapor generation rate and exhaust conditions is the dominant factor in determining the impact mode. Our results revealed more facts and features of the droplet impact phenomenon and can be very useful for target-oriented surface design towards precise control of droplet impact behavior on heated substrates. V C 2016 AIP Publishing LLC. [http://dx
Ring formation from drying sessile colloidal droplets (∼1.0 mm in size) containing microparticles of silicon or polystyrene was investigated with video microscopy. Results show that ring formation begins at the pinned contact line with the growth of an annular nucleus in a line by line way, which recedes inward albeit only slightly, followed by stacking of particles when the flow velocity becomes sufficiently large. The central height of the droplet decreases linearly with evaporation time, which implies that in the early stage, the number of particles arriving at contact line increases with time in a power law N∝t3/(1 + λ), where the parameter λ, according to Deegan's evaporation model, is related to the contact angle via $\lambda = \frac{{\pi - 2\theta _c }}{{2\pi - 2\theta _c }}$λ=π−2θc2π−2θc. Experimental values of λ agree well with model calculation for small contact angles, but are relatively smaller in the case of large contact angles. ‘Amorphization’ mechanism for the deposit at different stages of evaporation is discussed. Marangoni flow in a droplet on heated substrate introduces a desorption path for particles along the liquid surface, which can partially resolve the ring. Residual particles floating on the liquid surface may leave behind a homogeneous monolayer coating inside the dried spot. A “jump” in the droplet surface area at later stage of evaporation seems inevitably to cause a depletion zone of particles next to the ring. These results may be helpful for the development of strategies towards suppression of coffee ring effect and/or obtaining homogeneous coatings from drying colloidal suspension.
Deposition of colloidal particles in a drying droplet is important in many scientific researches and technological applications. In this work, the ring deposition of drying droplets on a solid substrate is investigated experimentally at a microscopic level. A ring deposition is formed at the contact line as the water solution droplet containing SiO2 particles is drying, just like the formation of coffee ring. Contact line pinning is crucial to the ring deposition formation. There will be a replenish flow in the droplet towards the edge, and the particles are driven to the contact line, deposited on the substrate. As the particle mass fraction is large, the particles which are left inside the spot, when the droplet dries out, may form a single particle layer, packing in order. The contact angle of the droplet on glass substrate is very small, the SiO2 particles will gather at the rim of the droplet, which initially form a chain along the contact line. As more particles come to the rim, they are deposited in a line by line way to form a 2D close packing. Since the contact angle decreases with evaporation when the contact line is pinned, a capillary force between liquid surface and particles arises once the height of droplet surface near the contact line is lower than that of the particle, pushing the particles to move inward. The effect on the larger particles is more pronounced-it even leads to a separation of the particles, with the smaller ones at the outer side.
Owing to its destructive power to porous structures such as buildings and rocks, salt weathering has attracted considerable attention in the community of civil engineers and geomorphologists, who devote their efforts to conservations of architecture and engineering structures afflicted by salt attack, and to the investigation of natural landscape caused by the same group of processes, respectively. Precipitation of dissolved salts is a direct cause of salt weathering effect. Crystallization phenomena in salt weathering can be crudely categorized under efflorescence and subflorescence with respect to the distinct precipitation sites, and the latter is believed to be able to cause more destructions to porous structure. In contrast to subflorescence for which even models of statistical dynamics have been well-established, efflorescence has drawn less attention, partly because of the complexity of constructing a sound theoretical model to describe the mass transport process there involved. As a serie of sodium salts is the main culprit of salt weathering, the current work deals with experimental study of efflorescences of the aqueous NaCl, NaNO3 and Na2SO4 solutions on the surface of porous silica gel particles. We investigate the influences of salt concentration and pore size on the crystal morphology arising in efflorescence by using scanning electron microscopy. It is found that though Na2SO4 effloresces on the specimen surface, its inclination towards subflorescence makes the whiskers appear on specimen with smaller pore radii at low concentrations, which differs obviously from the cases of NaCl and NaNO3. Moreover, unlike the upright growths of NaCl and NaNO3 crystals, the whiskers of Na2SO4 are always oblique to the specimen surface, and the large lateral stress to the specimen thus induced may become another factor of its destructive power apart from the subflorescing trend. The crystallization behaviors of Na2SO4, i.e., both the oblique whiskers and regular crystallites, indicate that mirabilite (Na2SO410H2O) is the main precipitation, which is consistent with the high relative humidity employed in this article. Remarkably, the thinnest whiskers of NaNO3 exhibit the branching and ball-chain structures, indicating that plateau-Rayleigh instablility occurs in the growth process. Our results are expected to inspire more deliberate studies for the full understanding of detailed processes and mechanism involved in efflorescence of aqueous salt solutions.
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