Evaporative Deposition of Surfactant-Laden Nanofluid Droplets over a Silicon Surface.

Morphologies of evaporative deposition, which has been widely applied in potential fields, were induced by the competition between internal flows inside evaporating droplets. Controlling the pattern of deposition and suppressing the coffee-ring effect are essential issues of intense interest in the aspects of industrial technologies and scientific applications. Here, evaporative deposition of surfactant-laden nanofluid droplets over silicon was experimentally investigated. A ring-like deposition was formed after complete evaporation of sodium dodecyl sulfate (SDS)-laden nanofluid droplets with an initial SDS concentration ranging from 0 to 1.5 CMC. In the case of initial SDS concentrations above 1.3 CMC, no cracks were observed in the ring-like deposition, indicating that the deposition patterns of nanofluid droplets could be completely changed and cracks could be eliminated by sufficient addition of SDS. With the increase of the initial concentration of hexadecyl trimethylammonium bromide (CTAB), the width of the deposition ring gradually decreased until no ring-like structure was formed. On the contrary, with the increase of the initial Triton X-100 (TX-100) concentration, the width of the deposition ring gradually increased until a uniform deposition was generated. Moreover, when the initial TX-100 concentration was high, a "tree-ring-like" pattern was discovered. Besides, morphologies of evaporative pattern due to the addition of surfacants were qualitatively analyzed.

Droplet impact on pillar-arrayed non-wetting surfaces.

Droplet impact on pillar-arrayed polydimethylsiloxane (PDMS) surfaces with different solid fractions was studied. The lower and upper limits of Weber number, We, for complete rebound of impacting droplets decreased with decreasing solid fractions. Gaps were visible during the spreading and retraction processes of bouncing droplets on the surface with a solid fraction of 0.06 while no gaps were observed during the retraction process when We was greater than its upper limit, indicating that there existed a transition from the Cassie-Baxter wetting state to the Wenzel wetting state. Therefore, a novel model accounting for the penetration of a liquid into the cavities between the pillars was developed to predict the upper limit of the impact velocity of bouncing droplets. At high We, partial rebound was observed for surfaces with solid fractions of 0.50 and 0.20 while a sticky state was observed for the surface with a solid fraction of 0.06. Moreover, surface roughness has a great influence on the contact time of bouncing droplets. Besides, the maximum spreading parameter was found to follow a scaling law of We1/4.

Author Correction: Evaporative deposition of polystyrene microparticles on PDMS surface.

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Evaporative deposition of polystyrene microparticles on PDMS surface.

Evaporation of water and ethanol/water droplets containing large polystyrene (PS) microparticles on polydimethylsiloxane (PDMS) surface was experimentally investigated. It is found that no matter with or without small addition of ethanol, a compact monolayer deposition is formed for lower microparticle concentration while mountain-like deposition for higher concentration. Since the more volatile compound (ethanol) evaporates more quickly than the less volatile compound (water), evaporation of ethanol/water mixture droplet exhibits different characteristics from pure water. When the concentration of microparticle is low, the contact radius of ethanol/water mixture droplet decreases throughout the whole process, while the contact angle increases at first to a maximum, then keeps almost constant, and finally decreases sharply. However, the evaporation of ethanol/water mixture droplet with higher concentration of microparticle behaviors more complex. The settling time of microparticles was estimated and its theoretical value agrees well with the experimental one. Moreover, a mechanism of self-pinning of microparticles was used to elucidate the deposition behavior of microparticles, indicating that as the contact line is depinning, the liquid film covering the outmost microparticle becomes thicker and thicker, and the microparticles have to move spontaneously with the depinning contact line under the action of capillary force.

Experimental and theoretical investigations of evaporation of sessile water droplet on hydrophobic surfaces.

Experiments of sessile water droplet evaporation on both polydimethylsiloxane (PDMS) and Teflon surfaces were conducted. All experiments begin with constant contact area mode (the initial contact angle is greater than 90°), switch to constant contact angle mode and end with mixed mode. Based on the assumptions of spherical droplet and uniform concentration gradient, theoretical analyses for both constant contact area and constant contact angle modes are made and theoretical solutions are derived accordingly, especially a theoretical solution of contact angle is presented first for CCR stage with any value of the initial contact angle. Moreover, comparisons between the theoretical solutions and experimental data of contact angle in CCR stage demonstrate the validity of the theoretical solution and it would help for a better understanding and application of water droplet on solid surfaces, which is quite often encountered in lab-on-a-chip, polymerase chain reaction (PCR) and other micro-fluidics devices.

Deformation of PDMS membrane and microcantilever by a water droplet: comparison between Mooney-Rivlin and linear elastic constitutive models.

In this paper, we studied the role of vertical component of surface tension of a water droplet on the deformation of membranes and microcantilevers (MCLs) widely used in lab-on-a-chip and micro- and nano-electromechanical system (MEMS/NEMS). Firstly, a membrane made of a rubber-like material, poly(dimethylsiloxane) (PDMS), was considered. The deformation was investigated using the Mooney-Rivlin (MR) model and the linear elastic constitutive relation, respectively. By comparison between the numerical solutions with two different models, we found that the simple linear elastic model is accurate enough to describe such kind of problem, which would be quite convenient for engineering applications. Furthermore, based on small-deflection beam theory, the effect of a liquid droplet on the deflection of a MCL was also studied. The free-end deflection of the MCL was investigated by considering different cases like a cylindrical droplet, a spherical droplet centered on the MCL and a spherical droplet arbitrarily positioned on the MCL. Numerical simulations demonstrated that the deflection might not be neglected, and showed good agreement with our theoretical analyses.

Elastic deformation of soft membrane with finite thickness induced by a sessile liquid droplet.

In this paper, the role of vertical component of surface tension of a droplet on the elastic deformation of a finite-thickness flexible membrane was theoretically analyzed using Hankel transformation. The vertical displacement at the surface was derived and can be reduced to Lester's or Rusanov's solutions when the thickness is infinite. Moreover, some simulations of the effect of a liquid droplet on a membrane with a finite thickness were made. The numerical results showed that there exists a saturated membrane thickness of the order of millimeter, when the thickness of a membrane is larger than such a value, the membrane can be regarded as a half-infinite body. Further numerical calculations for soft membrane whose thickness is far below the saturated thickness were made. By comparison between the maximum vertical displacement of an ultrathin soft membrane and a half-infinite body, we found that Lester's or Rusanov's solutions for a half-infinite body cannot correctly describe such cases. In other words, the thickness of a soft membrane has great effect on the surface deformation of the ultrathin membrane induced by a liquid droplet.