Dynamic Chemically Driven Dewetting, Spreading, and Self-Running of Sessile Droplets on Crystalline Silicon

Abstract

A chemically driven dewetting effect is demonstrated using sessile droplets of dilute hydrofluoric acid on chemically oxidized silicon wafers. The dewetting occurs as the thin oxide is slowly etched by the droplet and replaced by a hydrogen-terminated surface; the result of this is a gradual increase in the contact angle of the droplet with time. The time-varying work of adhesion is calculated from the time-varying contact angle; this corresponds to the changing chemical nature of the surface during dewetting and can be modeled by the well-known logistic (sigmoid) function often used for the modeling of restricted growth, in this case, the transition from an oxidized surface to a hydrogen-terminated silicon surface. The observation of the time-varying contact angle allows one to both measure the etch rate of the silicon oxide and estimate the hydrogenation rate as a function of HF concentration and wafer type. In addition to this, at a certain HF concentration, a self-running droplet effect is observed. In contrast, on hydrogen-terminated silicon wafers, a chemically induced spreading effect is observed using sessile droplets of nitric acid. The droplet spreading can also be modeled using a logistical function, where the restricted growth is the transition from hydrogen-terminated to a chemically induced oxidized silicon surface. The chemically driven dewetting and spreading observed here add to the methods available to study dynamic wetting (e.g., the moving three-phase contact line) of sessile droplets on surfaces. By slowing down chemical kinetics of the wetting, one is able to record the changing profile of the sessile droplet with time and gather information concerning the time-varying surface chemistry. The data also indicates a chemical interface hysteresis (CIH) that is compared to contact angle hysteresis (CAH). The approach can also be used to study the chemical etching and deposition behavior of thin films using liquids by monitoring the macroscopic droplet profile and relating this to the time-varying physical and chemical interface phenomena.

When is a surface foam-phobic or foam-philic?

“…These five surfaces were prepared to ensure a range of wetting properties (with respect to the surfactant solution) from hydrophilic (low contact angle) to hydrophobic (high contact angle). A hydrophilic surface was prepared by chemically oxidising a commercial p-type (5 − 10 Ωcm) polished silicon wafer (Siltronix, France) in 65% nitric acid, thus creating a thin silicon oxide layer having a thickness of approximately 1 nm [12] (surface 1). Three intermediate wetting surfaces were fabricated from a commercial polished silicon wafer (roughness < 1 nm) coated with a thin amorphous fluorocarbon (FC) layer [13] -referred to here as 'teflonised polished silicon' (surface 2); a 1 mm thick polydimethylsiloxane 'PDMS' elastomer block (1:10 PDMS Sylgard 184 Dow Corning) moulded in a dish (surface 3); and a 'teflonised rough silicon' surface made by depositing the thin amorphous FC on the unpolished rear side of a commercial silicon wafer of roughness ≈ 1 µm (surface 4).…”

“…A similar analysis can be performed to determine the validity of equations (12) and (16) for the top Plateau border. Note that now the z ′ -axis is directed downwards from z ′ = 0 (the top substrate).…”

“…First, it is instructive to consider the case Bo = 0, corresponding to zero gravity, for which the Plateau borders at the top and bottom substrates behave identically: their surfaces are arcs of circle. The integrals in equations ( 11) or (12), and ( 15) or ( 16) can now be performed analytically, yielding…”

“…Although, as for the bottom Plateau border, equation (12) can still be solved in the shaded domain of figure 9b, the resulting Plateau borders are unphysical. In the crosshatched domain, which is a mirror image of that found for the bottom Plateau border, the Young-Laplace equation has no solution.…”

Electrowetting of Hydrofluoroether Liquid Droplet at a Gold Electrode/Water Interface: Significance of Lower Adhesion Energy and Static Friction Energy

“…Arscott demonstrated a spontaneous movement of a HF aqueous solution droplet on the silicon wafer surface in air in the configuration of Figure 1b. 27 An inhomogeneous increase of θ is induced by a progressive chemical oxidation of H-terminated silicon surface by HF. Note that these redox-driven locomotions are not repeatable but have limited durability.…”