Wetting Dynamics on Solvophilic, Soft, Porous, and Responsive Surfaces

Abstract

We employ molecular dynamics (MD) simulations to study the spreading and imbibition of a liquid drop on a porous, soft, solvophilic, and responsive surface represented by a layer of polymer molecules grafted on a solvophilic solid. These polymer molecules are in a crumpled and collapsed globule-like state before the interaction with the drop but transition to a “brush”-like state as they get wetted by the liquid drop. We hypothesize that for a wide range of densities of polymer grafting (σg), the drop spreading is dictated by the balance of the driving inertial pressure and balancing viscoelastic dissipation (associated with the spreading of the liquid drop on the polymer layer that undergoes globule-to-brush transition and serves as the viscoelastic solid). Using the well-known idea that the viscoelastic resisting force exerted by the viscoelastic solid on a spreading drop scales as u n (where n is the index of the power-law-like rheology of the polymer layer serving as the viscoelastic solid and u is the spreading velocity of the drop on this viscoelastic solid) and considering n = 2/3, we show that the scaling calculation recovers the MD simulation prediction of r ∼ t 1/4 and r eq ∼ σg –1/3 (where r and r eq are the instantaneous and equilibrium spreading radii, respectively). We further describe the wicking behavior of the drop through the polymer layer by appropriately accounting for the manner in which the progressive time-dependent swelling of the grafted polymer molecules provides larger space for the wicking. Third, we quantify, possibly for the first time, the temporal dynamics of the “brush”-forming process (i.e., capture the dynamics of wetting-mediated globule-to-brush transition). We show that the dynamics of the polymer chain swelling depends on σg and is faster for sparser grafting. Most importantly, we confirm that the height of the relaxed polymer chains approximately scales as σg 1/3, confirming the attainment of brush-like configuration by the polymer molecules as they are wetted by the liquid drop. Finally, we argue that our simulations raise the possibility of designing soft, “responsive”, and widely deployable liquid-infused surfaces where the polymer grafted solid, with the polymer undergoing a globule-to-brush transition, serves as the responsive “surface”.

Cloaking Transition of Droplets on Lubricated Brushes

“…This will be the subject of future investigations. Another interesting subject for future investigation is the spreading and imbibition of the free chains into the brush and comparison with atomistic simulations …”

“…Another interesting subject for future investigation is the spreading and imbibition of the free chains into the brush and comparison with atomistic simulations. 46 ■ APPENDIX A: EXPERIMENTAL DETAILS PDMS Brush Substrate Glass slides (24 × 60 mm, 170 μm thick) were rinsed with ethanol and acetone and dried under nitrogen gas. Thereafter, they were exposed to oxygen plasma (0.3 bar) for 10 min at 300 W (Femto, Diener Electronic, Germany).…”

Fundamentals and Applications of Polymer Brushes in Air

“…They identified a time scaling r ∼ t 1/4 and an equilibrium drop radius of r eq ∼ ρ g –1/3 , consistent with a scaling approach in which the capillary driving force is balanced by viscoelastic forces resulting from the drop-substrate interaction. 218 Moreover, they investigate the swelling dynamics of polymer chains as a function of grafting density via the brush height in the early stages of the wetting process, and find that the initial swelling response follows an approximate power law h ∼ t δ , where the exponent δ is typically smaller than unity and decreases as the grafting density of the brush increases, resulting in the intuitive conclusion that denser brushes display a slower swelling response. Thiele and Hartmann developed a model for the spreading of a drop on a polymer brush, based on gradient dynamics on a free energy expression accounting for capillary effects, brush wetting and brush elasticity.…”

Extensive Stable Physical Contacts between a Nanoparticle and a Highly Repulsive Polymeric Layer

“…On the other hand, the polymer molecules being philic to the drop (since ε LP = 1), the drop simultaneously spreads and imbibes within the polymer layer and in response the polymer molecules start stretching out as the solvent around them changes from “poor” (case of no solvent) to “good” (case of the solvent as the liquid to which the polymers are philic). This combined response of the drop (simultaneous spreading and imbibition) and the grafted polymer molecules (changing from collapsed to stretched states) has been studied in great detail in our previous paper . The most interesting question here is what happens to the NP as the drop spreads and imbibes through the grafted polymer layer?…”

“…This combined response of the drop (simultaneous spreading and imbibition) and the grafted polymer molecules (changing from collapsed to stretched states) has been studied in great detail in our previous paper. 22 The most interesting question here is what happens to the NP as the drop spreads and imbibes through the grafted polymer layer? we find that the NP starts to develop finite stable contacts with the polymer molecules: these contacts are direct "physical contacts" between the polymer and the NP without any intervening liquid molecules.…”