Drops on soft surfaces learn the hard way

Squires, Todd M.

doi:10.1073/pnas.1310672110

Cited by 7 publications

(3 citation statements)

References 24 publications

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“…The typical approach in the production of superhydrophobic surfaces is increasing surface roughness, through chemical modifications [27] and tuning mechanical properties of materials, such as stiffness [28]. The first one, roughness, can be controlled on electrospun surfaces via decorating of fiber surfaces [29], wrinkling [30,31], introducing core-shell [32] or core-sheath structures [33], and various surface treatment methods [34,35], which brings us to the second case.…”

Section: Introductionmentioning

confidence: 99%

Roughness and Fiber Fraction Dominated Wetting of Electrospun Fiber-Based Porous Meshes

et al. 2018

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Wettability of electrospun fibers is one of the key parameters in the biomedical and filtration industry. Within this comprehensive study of contact angles on three-dimensional (3D) meshes made of electrospun fibers and films, from seven types of polymers, we clearly indicated the importance of roughness analysis. Surface chemistry was analyzed with X-ray photoelectron microscopy (XPS) and it showed no significant difference between fibers and films, confirming that the hydrophobic properties of the surfaces can be enhanced by just roughness without any chemical treatment. The surface geometry was determining factor in wetting contact angle analysis on electrospun meshes. We noted that it was very important how the geometry of electrospun surfaces was validated. The commonly used fiber diameter was not necessarily a convincing parameter unless it was correlated with the surface roughness or fraction of fibers or pores. Importantly, this study provides the guidelines to verify the surface free energy decrease with the fiber fraction for the meshes, to validate the changes in wetting contact angles. Eventually, the analysis suggested that meshes could maintain the entrapped air between fibers, decreasing surface free energies for polymers, which increased the contact angle for liquids with surface tension above the critical Wenzel level to maintain the Cassie-Baxter regime for hydrophobic surfaces.

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Section: Introductionmentioning

confidence: 99%

Roughness and Fiber Fraction Dominated Wetting of Electrospun Fiber-Based Porous Meshes

et al. 2018

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“…Due to these merits, many studies have been performed with regard to droplet control 1 . For instance, passive methods utilizing channel geometry 2 – 4 , magnetic forces 5 – 7 , electrowetting-on-dielectrics (EWOD) 8 – 10 , optical tweezers 11 – 13 , and Marangoni effects 14 – 16 have been presented as means of droplet control.…”

Section: Introductionmentioning

confidence: 99%

Estimation of the thermocapillary force and its applications to precise droplet control on a microfluidic chip

Won

Lee

Song

2017

Sci Rep

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Droplet control through the use of light-induced thermocapillary effects has recently garnered attention due to its non-intrusive and multifunctional nature. An important issue in droplet control is the estimation of the thermocapillary force. The purpose of the present study is to estimate the thermocapillary force and propose empirical equations between the force and simply measurable key parameters such as droplet diameter and power of heat source. In addition, we aim to shift the droplet trajectory and develop an on-demand droplet routing system based on the estimation of the thermocapillary force. We illuminated a continuous phase with a 532 nm laser beam to minimize possible damage or property changes to target molecules contained within droplets. A mixture of light-absorbing material and oleic acid was used for the continuous phase fluid, while deionized water (DI water) was used for the dispersed phase fluid. We proposed empirical equations to estimate the thermocapillary force, which was then applied to precise droplet shifting and routing. We found that the shifting distance was linearly proportional to the thermocapillary force, and that an on-demand droplet routing system resulted in a success rate greater than 95%.

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“…Therefore, analytical and numerical modeling methods must be used to better understand the details of the evaporation mechanism that is relevant for sedimenting droplets. Although modeling of free droplets does not face complexities due to substrate–droplet interactions that control the droplet shape at the solid/liquid/air interface 35,36 and Marangoni flow, 37,38 which occur in the case of sessile droplets, other factors such as evaporation-induced concentration gradients inside the droplet 39 and the possibility of crust formation on the droplet surface, 40 which are consequences of the increasing solute surface concentration during evaporation, 41 cause difficulties even in the modeling of free droplets in the presence of solutes. In addition, physical and chemical properties of the drying droplets, such as the internal viscosity, 42,43 the diffusivity of solvent and solutes in the liquid phase, 43 and the activity coefficient of the solvent, 44 are dependent on the local concentration of solutes (and, consequently, on both position and time), which makes the problem rather complex.…”

Section: Introductionmentioning

confidence: 99%

Water evaporation from solute-containing aerosol droplets: Effects of internal concentration and diffusivity profiles and onset of crust formation

Rezaei

Netz

2021

Physics of Fluids

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The evaporation of droplets is an important process not only in industrial and scientific applications, but also in the airborne transmission of viruses and other infectious agents. We derive analytical and semi-analytical solutions of the coupled heat and mass diffusion equations within a spherical droplet and in the ambient vapor phase that describe the evaporation process of aqueous free droplets containing nonvolatile solutes. Our results demonstrate that the solute-induced water vapor-pressure reduction considerably slows down the evaporation process and dominates the solute-concentration dependence of the droplet evaporation time. The evaporation-induced enhanced solute concentration near the droplet surface, which is accounted for using a two-stage evaporation description, is found to further slow-down the drying process. On the other hand, the presence of solutes is found to produce a lower limit for the droplet size that can be reached by evaporation and, also, to reduce evaporation cooling of the droplet, which tend to decrease the evaporation time. Overall, the first two effects are dominant, meaning that the droplet evaporation time increases in the presence of solutes. Local variation of the water diffusivity inside the droplet near its surface, which is a consequence of the solute-concentration dependence of the diffusion coefficient, does not significantly change the evaporation time. Crust formation on the droplet surface increases the final equilibrium size of the droplet by producing a hollow spherical particle, the outer radius of which is determined as well.

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Drops on soft surfaces learn the hard way

Cited by 7 publications

References 24 publications

Roughness and Fiber Fraction Dominated Wetting of Electrospun Fiber-Based Porous Meshes

Roughness and Fiber Fraction Dominated Wetting of Electrospun Fiber-Based Porous Meshes

Estimation of the thermocapillary force and its applications to precise droplet control on a microfluidic chip

Water evaporation from solute-containing aerosol droplets: Effects of internal concentration and diffusivity profiles and onset of crust formation

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