Stick-Slip Sliding of Water Drops on Chemically Heterogeneous Surfaces

Varagnolo, Silvia; Ferraro, Davide; Fantinel, Paolo; Pierno, Matteo; Mistura, Giampaolo; Amati, Giorgio; Biferale, Luca; Sbragaglia, Mauro

doi:10.1103/physrevlett.111.066101

Cited by 136 publications

(161 citation statements)

References 34 publications

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“…At this small capillary number, capillary fingers form and break up, without leaving visible droplets in the hydrophilic regions. Such stick-slip behavior of a moving contact line on chemically patterned surfaces has been observed before both computationally (41) and experimentally (42). The stick-slip capillary finger formation and breakup is captured in the advancing and receding force-displacement curves as sawtooth patterns.…”

Section: Resultsmentioning

confidence: 74%

Spontaneous wettability patterning via creasing instability

Chen

McKinley

Cohen

2016

Proc. Natl. Acad. Sci. U.S.A.

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Surfaces with patterned wettability contrast are important in industrial applications such as heat transfer, water collection, and particle separation. Traditional methods of fabricating such surfaces rely on microfabrication technologies, which are only applicable to certain substrates and are difficult to scale up and implement on curved surfaces. By taking advantage of a mechanical instability on a polyurethane elastomer film, we show that wettability patterns on both flat and curved surfaces can be generated spontaneously via a simple dip coating process. Variations in dipping time, sample prestress, and chemical treatment enable independent control of domain size (from about 100 to 500 μm), morphology, and wettability contrast, respectively. We characterize the wettability contrast using local surface energy measurements via the sessile droplet technique and tensiometry.wettability contrast | creasing instability | domain size | morphology | curved surfaces S urfaces that juxtapose local hydrophilic areas with hydrophobic areas show superior performance compared with surfaces with homogeneous wettability in many industrial applications including heat transfer (1), water collecting (2-6), particle separation (7), and microfluidics (8). For instance, developing enhanced water-collecting efficiency has been inspired by the Namib desert beetle, which was reported (2) to have hydrophilic bumps on an overall wax-covered hydrophobic surface. Although the hydrophilic bumps reduce the nucleation/coalescence energy of microdroplets, the overall hydrophobic character of the surface facilitates the spontaneous shedding of water droplets when they grow beyond a certain size.To achieve surfaces with wettability patterning, traditional fabrication methods such as photolithography and soft lithography have been used. However, these methods are generally not cost-effective, not readily scaled up, require multiple process steps, and are difficult to implement on curved surfaces (3, 9). Recently, mechanical instabilities have been explored as a facile self-assembly approach to endow surfaces with superhydrophobicity (10-12), superhydrophilicity (13), or anisotropic wettability (14). Although mechanical self-assembly provides a low cost route for spontaneous generation of surface patterns, explorations to date have been focused on introducing surface roughness via wrinkling and crumpling instabilities. Here we show that surfaces can be spontaneously patterned with chemical patches with small changes in surface roughness by harnessing a reversible creasing instability (15-17).A creasing instability develops when a soft polymer network is placed under mechanical compression beyond a certain critical strain, at which point sharp folds spontaneously develop and grow on the deformable free surfaces (18). This process has been linked to the morphology development of the brain (19,20), electric breakdown of dielectric elastomers (21), and has also been harnessed to prepare switchable surfaces actuated by temperature (22) or electric ...

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Section: Resultsmentioning

confidence: 74%

Spontaneous wettability patterning via creasing instability

Chen

McKinley

Cohen

2016

Proc. Natl. Acad. Sci. U.S.A.

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“…For a better understanding of these phenomena, it would be of great interest to complement the experimental observations with numerical simulations that allow to evaluate both the stress tensor and the distribution of the shear rate inside the T-junction, and to monitor the momentum balance equations for non-Newtonian fluids during the process of break-up. To this aim, generalization of the investigations we have proposed for sliding droplets [41,42] are surely warranted for future investigations.…”

Section: Discussionmentioning

confidence: 99%

Generation of Oil Droplets in a Non-Newtonian Liquid Using a Microfluidic T-Junction

et al. 2015

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Abstract:We have compared the formation of oil drops in Newtonian and non-Newtonian fluids in a T-junction microfluidic device. As Newtonian fluids, we used aqueous solutions of glycerol, while as non-Newtonian fluids we prepared aqueous solutions of xanthan, a stiff rod-like polysaccharide, which exhibit strong shear-thinning effects. In the squeezing regime, the formation of oil droplets in glycerol solutions is found to scale with the ratio of the dispersed flow rate to the continuous one and with the capillary number associated to the continuous phase. Switching to xanthan solutions does not seem to significantly alter the droplet formation process. Any quantitative difference with respect to the Newtonian liquid can be accounted for by a suitable choice of the capillary number, corresponding to an effective xanthan viscosity that depends on the flow rates. We have deduced ample variations in the viscosity, on the order of 10 and more, during normal operation conditions of the T-junction. This allowed estimating the actual shear rates experienced by the xanthan solutions, which go from tens to hundreds of s´1.

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“…In most cases, the substrates on which droplets spread are not smooth but are characterized by various complexities such as chemical and/or topographical variationsheterogeneities which can give rise to very rich dynamics characterized by, e.g., hysteresis behavior and stick-slip motion. [1][2][3][4][5][6][7][8][9][10][11] Examples of complex substrates also include the case of a membrane or a porous medium, where, in addition to the intrinsic properties of the material, one needs to take into account the fact that the volume of the droplet may vary in time. In particular, an important feature here is that liquid can be absorbed or pumped in through the membrane, 12,13 i.e.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of Fattening and Thinning 2D Sessile Droplets

Pradas

Savva

Benziger³

et al. 2016

Langmuir

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We investigate the dynamics of a droplet on a planar substrate as the droplet volume changes dynamically due to liquid being pumped in or out through a pore. We adopt a diffuse-interface formulation which is appropriately modified to account for a localized inflow-outflow boundary condition (the pore) at the bottom of the droplet, hence allowing to dynamically control its volume, as the droplet moves on a flat substrate with a periodic chemical pattern. We find that the droplet undergoes a stick-slip 1 motion as the volume is increased (fattening droplet) which can be monitored by tracking the droplet contact points. If we then switch over to outflow conditions (thinning droplet) the droplet follows a different path (i.e. the distance of the droplet midpoint from the pore location evolves differently) giving rise to a hysteretic behavior. By means of geometrical arguments we are able to theoretically construct the full bifurcation diagram of the droplet equilibria (positions and droplet shapes) as the droplet volume is changed, finding excellent agreement with time-dependent computations of our diffuse-interface model.

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Stick-Slip Sliding of Water Drops on Chemically Heterogeneous Surfaces

Cited by 136 publications

References 34 publications

Spontaneous wettability patterning via creasing instability

Spontaneous wettability patterning via creasing instability

Generation of Oil Droplets in a Non-Newtonian Liquid Using a Microfluidic T-Junction

Dynamics of Fattening and Thinning 2D Sessile Droplets

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