scholarly journals Classifying dynamic contact line modes in drying drops

Soft Matter ◽  
2015 ◽  
Vol 11 (8) ◽  
pp. 1628-1633 ◽  
Author(s):  
Kyle Anthony Baldwin ◽  
David John Fairhurst
Keyword(s):  
Contact Line ◽  
Polymer Solutions ◽  
Dynamic Contact ◽  
Low Pressure ◽  
Droplet Radius ◽  
Dynamic Contact Line ◽  
Scaling Arguments ◽  
Drying Drops

Evaporating droplets of both PEO polymer solutions and blood at low pressure are observed to form pillar-like deposits when dried. We use normalised h–R plots to illustrate that this behaviour is volume-independent and find scaling arguments to support our measurements that the contact line recedes with a speed inversely proportional to the droplet radius.

Marangoni-enhanced capillary wetting in surfactant-driven superspreading

2018 ◽  
Vol 855 ◽  
pp. 181-209 ◽  
Author(s):  
Hsien-Hung Wei
Keyword(s):  
Contact Line ◽  
Stress Singularity ◽  
Marangoni Effect ◽  
Dynamic Contact ◽  
Sorption Kinetics ◽  
Colloid Interface ◽  
Hydrodynamic Approach ◽  
Interfacial Surfactant ◽  
Dynamic Contact Line ◽  
Air Liquid Interface

Superspreading is a phenomenon such that a drop of a certain class of surfactant on a substrate can spread with a radius that grows linearly with time much faster than the usual capillary wetting. Its origin, in spite of many efforts, is still not fully understood. Previous modelling and simulation studies (Karapetsas et al. J. Fluid Mech., vol. 670, 2011, pp. 5–37; Theodorakis et al. Langmuir, vol. 31, 2015, pp. 2304–2309) suggest that the transfer of the interfacial surfactant molecules onto the substrate in the vicinity of the contact line plays a crucial role in superspreading. Here, we construct a detailed theory to elaborate on this idea, showing that a rational account for superspreading can be made using a purely hydrodynamic approach without involving a specific surfactant structure or sorption kinetics. Using this theory it can be shown analytically, for both insoluble and soluble surfactants, that the curious linear spreading law can be derived from a new dynamic contact line structure due to a tiny surfactant leakage from the air–liquid interface to the substrate. Such a leak not only establishes a concentrated Marangoni shearing toward the contact line at a rate much faster than the usual viscous stress singularity, but also results in a microscopic surfactant-devoid zone in the vicinity of the contact line. The strong Marangoni shearing then turns into a local capillary force in the zone, making the contact line in effect advance in a surfactant-free manner. This local Marangoni-driven capillary wetting in turn renders a constant wetting speed governed by the de Gennes–Cox–Voinov law and hence the linear spreading law. We also determine the range of surfactant concentration within which superspreading can be sustained by local surfactant leakage without being mitigated by the contact line sweeping, explaining why only limited classes of surfactants can serve as superspreaders. We further show that spreading of surfactant spreaders can exhibit either the $1/6$ or $1/2$ power law, depending on the ability of interfacial surfactant to transfer/leak to the bulk/substrate. All these findings can account for a variety of results seen in experiments (Rafai et al. Langmuir, vol. 18, 2002, pp. 10486–10488; Nikolov & Wasan, Adv. Colloid Interface Sci., vol. 222, 2015, pp. 517–529) and simulations (Karapetsas et al. 2011). Analogy to thermocapillary spreading is also made, reverberating the ubiquitous role of the Marangoni effect in enhancing dynamic wetting driven by non-uniform surface tension.

scholarly journals Numerical simulation of dynamic contact-line problems

1997 ◽  
Vol 352 ◽  
pp. 113-133 ◽  
Author(s):  
IVAN B. BAZHLEKOV ◽  
PETER J. SHOPOV
Keyword(s):  
Contact Line ◽  
Contact Region ◽  
Outer Region ◽  
Dynamic Contact ◽  
Momentum Conservation ◽  
Phase Region ◽  
Contact Lines ◽  
Three Phase ◽  
Inner Region ◽  
Dynamic Contact Line

The presence of a three-phase region, where three immiscible phases are in mutual contact, causes additional difficulties in the investigation of many fluid mechanical problems. To surmount these difficulties some assumptions or specific hydrodynamic models have been used in the contact region (inner region). In the present paper an approach to the numerical solution of dynamic contact-line problems in the outer region is described. The influence of the inner region upon the outer one is taken into account by means of a solution of the integral mass and momentum conservation equations there. Both liquid–fluid–liquid and liquid–fluid–solid dynamic contact lines are considered. To support the consistency of this approach tests and comparisons with a number of experimental results are performed by means of finite-element numerical simulations.

Surface-Tension-Driven Dynamic Contact Line in Microgravity

Langmuir ◽  
2018 ◽  
Vol 35 (2) ◽  
pp. 406-412 ◽  
Author(s):  
Péter Bába ◽  
Ágota Tóth ◽  
Dezső Horváth
Keyword(s):  
Surface Tension ◽  
Contact Line ◽  
Dynamic Contact ◽  
Dynamic Contact Line

Mechanisms of dynamic wetting failure in the presence of soluble surfactants

2017 ◽  
Vol 825 ◽  
pp. 677-703 ◽  
Author(s):  
Chen-Yu Liu ◽  
Marcio S. Carvalho ◽  
Satish Kumar
Keyword(s):  
Contact Angle ◽  
Contact Line ◽  
Rectangular Channel ◽  
Dynamic Contact ◽  
Slip Boundary Condition ◽  
Dynamic Wetting ◽  
Navier Slip ◽  
Static Contact ◽  
Soluble Surfactants ◽  
Dynamic Contact Line

A hydrodynamic model and flow visualization experiments are used to understand the mechanisms through which soluble surfactants can influence the onset of dynamic wetting failure. In the model, a Newtonian liquid displaces air in a rectangular channel in the absence of inertia. A Navier-slip boundary condition and constant contact angle are used to describe the dynamic contact line, and surfactants are allowed to adsorb to the interface and moving channel wall (substrate). The Galerkin finite element method is used to calculate steady states and identify the critical capillary number $Ca^{crit}$ at which wetting failure occurs. It is found that surfactant solubility weakens the influence of Marangoni stresses, which tend to promote the onset of wetting failure. Adsorption of surfactants to the substrate can delay the onset of wetting failure due to the emergence of Marangoni stresses that thicken the air film near the dynamic contact line. The experiments indicate that $Ca^{crit}$ increases with surfactant concentration. For the more viscous solutions used, this behaviour can largely be explained by accounting for changes to the mean surface tension and static contact angle produced by surfactants. For the lowest-viscosity solution used, comparison between the model predictions and experimental observations suggests that other surfactant-induced phenomena such as Marangoni stresses may play a more important role.

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