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45063-G9
The Interfacial Structure of Water on a Hydrophobic Surface: A Mechanistic Study

Yingxi Elaine Zhu, University of Notre Dame

            This research aims to understand the interfacial molecular structure and water transport at varied flow rates near the surfaces of varied hydrophobicity by complementary force-sensing and microscopic approaches.

Recent Progress

            In the first year of the grant, we have made some notable progress in characterizing and understanding the self-assembled structure and water transport properties of polymer aqueous solutions at the water-solid interface upon evaporation. We have also investigated the dynamic responses of water confined at Janus interfaces of varied hydrophobicity. Our key accomplishments are highlighted below.

            To explore the interfacial structure at the water-surface interface, we study the morphology of polymer stain from evaporating a polymer aqueous droplet at surface by concurrent microscopic visualization and contact angle measurement. We focus on aqueous solutions of DNA as it precipitates readily near a surface and also allows high-resolution imaging with fluorescent tags. In sharp contrast to the well-known coffee-stain pattern that results from a pinned contact line upon drying, we observed multi-ring stain patterns at different DNA concentration, temperature and surface wettability that all influence the stain formation as shown in Fig. 1a. We further explore the discontinuous pinning-depinning mechanism against DNA concentrations, where an apparent stick-slip dynamics has been endowed by the rupturing and pinning sequence. The measured ring wavelength strongly depends on DNA concentration as shown in Fig. 1b and similar multi-ring stains are also observed on amine-and methyl-terminated self-assembled monolayers, suggesting that precipitation dynamics, not evaporation rate nor surface tension, control the unsteady drop motion at surface. The existence of a surface stagnation flow within the evaporating drop leading to the precipitation of DNA macromolecules at the stagnation region is evident from fluorescent micrographs as shown in Fig. 1c, accompanied by the oscillations in measured contact angle of aqueous sessile drops at surface shown in Fig. 1d. The existence of the inner ring is also supported by the observation of miscible viscous fingers showing an approximate square wave pattern, which occurs when a less viscous fluid is pushing against a more viscous fluid, due to the difference in DNA concentration between outer and inner rings.

We have also developed a simple mass balance model to explain the scaling for the stain spacings, λ that is inversely proportional to the bulk DNA concentration. Based on 1-D flux balance of precipitate polymer at the stagnation ring, the spacing between two successive rings is consequently derived as λ ≈ uτ ≈ Δx C_m/C_∞, where u is the local evaporative flux, C_m is the maximum concentration of DNA achieved within the precipitate ring of width Δx in time-scale, τ and C_∞ is the bulk polymer concentration. This scaling relation appears strikingly consistent with our experimental results. The generality is examined with sub-micron sized polystyrene colloidal suspensions and similar precipitation dynamics is confirmed. The pattern formation by drying polymers at water-solid interfaces is quite rich and can yield a variety of stain patterns for self-assembly and small-scale fabrication processes. A manuscript was recently submitted to Phys. Rev. Lett.

Separately, we have studied the dynamic response of interfacial water of 1-2 nanometers thick confined between one partially hydrophobic surface and the other highly hydrophilic surface by using a modified surface forces apparatus. Earlier work shows giant fluctuation in shear response with confined water at a Janus aqueous interface. In this work, we confirm an everlasting extraordinary fluctuation in the absence of shear, whose amplitude diminishes with decreasing surface hydrophobicity. Consistently, a transition from a gel-like structure to a liquid-like one for ultrathin aqueous films is observed when surface hydrophobicity is reduced to water contact angle of less than 70 degrees. These observations suggest that the flicking capillary-like interface can be suppressed by enhancing water-surface interaction. A manuscript is being prepared.

Future Plans

            The above-described research areas are currently being continued and extended into new directions. Our overarching goal is an improved understanding, at a microscopic level, of the effects of flow rates and surface hydrophobicity on the interfacial structure of water near a solid surface by direct and non-invasive experimental approach. In the second year of the grant, we will conduct the following tasks: 1) to characterize the self-assembled structure of drying polymer-colloid binary aqueous solutions at surface and to explore how the pattern formation is disturbed by competing polymer-colloidal interactions and interfacial interactions; 2) to actively control the stick-slip dynamics of drying aqueous droplets by varying chemical and topographical patchiness of the solid surfaces; 3) to explore the effect of dissolved gas on the self-assembled structure of polymer stain at the water-solid interface.

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