Siang-Jie Hong

Superhydrophilicity to superhydrophobicity transition of CuO nanowire films

The surface of CuO is known for its hydrophilicity and exhibits superhydrophilic nature as nanowires are present. When exposed in the air at room temperature or treated by low temperature annealing, however, transition from superhydrophilicity to superhydrophobicity of the CuO nanowire films are observed. Since the chemical structure of the films after treatment remains the same as CuO according to x-ray photoelectron spectroscopy spectra, the superhydrophobicity may be attributed to partial deoxidation of the upmost layer of CuO surfaces into Cu2O-like hydrophobic surfaces. Nonetheless, superhydrophilicity is recovered if the superhydrophobic CuO film is subject to high temperature annealing.

High contact angle hysteresis of superhydrophobic surfaces: Hydrophobic defects

A typical superhydrophobic surface is essentially nonadhesive and exhibits very low water contact angle (CA) hysteresis, so-called Lotus effect. However, leaves of some plants such as scallion and garlic with an advancing angle exceeding 150° show very serious CA hysteresis. Although surface roughness and epicuticular wax can explain the very high advancing CA, our analysis indicates that the unusual hydrophobic defect, diallyl disulfide, is the key element responsible for contact line pinning on allium leaves. After smearing diallyl disulfide on an extended polytetrafluoroethylene (PTFE) film, which is originally absent of CA hysteresis, the surface remains superhydrophobic but becomes highly adhesive.

Anomalous Contact Angle Hysteresis of a Captive Bubble: Advancing Contact Line Pinning

Contact angle hysteresis of a sessile drop on a substrate consists of continuous invasion of liquid phase with the advancing angle (θ(a)) and contact line pinning of liquid phase retreat until the receding angle (θ(r)) is reached. Receding pinning is generally attributed to localized defects that are more wettable than the rest of the surface. However, the defect model cannot explain advancing pinning of liquid phase invasion driven by a deflating bubble and continuous retreat of liquid phase driven by the inflating bubble. A simple thermodynamic model based on adhesion hysteresis is proposed to explain anomalous contact angle hysteresis of a captive bubble quantitatively. The adhesion model involves two solid–liquid interfacial tensions (γ(sl) > γ(sl)′). Young’s equation with γ(sl) gives the advancing angle θ(a) while that with γ(sl)′ due to surface rearrangement yields the receding angle θ(r). Our analytical analysis indicates that contact line pinning represents frustration in surface free energy, and the equilibrium shape corresponds to a nondifferential minimum instead of a local minimum. On the basis of our thermodynamic model, Surface Evolver simulations are performed to reproduce both advancing and receding behavior associated with a captive bubble on the acrylic glass.

Equilibrium Phase Diagram of Drop-on-Fiber: Coexistent States and Gravity Effect

Drop-on-fiber is commonly observed in daily life and is closely related to digital microfluidics. The wetting behavior of droplet-on-fiber is different from that of droplet-on-plane due to the global cylindrical shape. It is generally believed that the equilibrium geometric shape of a droplet on a fiber takes either asymmetric clam-shell or axisymmetric barrel conformation in the absence of gravity. The barrel-to-clam-shell transition is determined by the stability condition. Nonetheless, experimental observations showed that both barrel and clam-shell conformations can coexist in some situations and thus indicated the existence of the multiple stable states. In this Article, the phase diagrams of droplet-on-fiber, that is, the plots of droplet volume against contact angle, are established on the basis of the finite-element simulation (Surface Evolver). When the gravity effect is absent, there are three regimes including barrel, clam-shell, and coexistence of barrel and clam-shell. As the gravity effect is considered, there exist three regimes, including (I) downward clam-shell, (II) coexistence of barrel and clam-shell, and (III) falling-off.

Drops Sitting on a Tilted Plate: Receding and Advancing Pinning

The wetting behavior of a liquid drop sitting on an inclined plane is investigated experimentally and theoretically. Using Surface Evolver, the numerical simulations are performed based on the liquid-induced defect model, in which only two thermodynamic parameters (solid-liquid interfacial tensions before and after wetting) are required. A drop with contact angle (CA) equal to θ is first placed on a horizontal plate, and then the plate is tilted. Two cases are studied: (i) θ is adjusted to the advancing CA (θ(a)) before tilting, and (ii) θ is adjusted to the receding CA (θ(r)) before tilting. In the first case, the uphill CA declines and the downhill CA remains unchanged upon inclination. When the tilted drop stays at rest, the pinning of the receding part of the contact line (receding pinning) and the depinning of the advancing part of the contact line (advancing depinning) are observed. The free energy analysis reveals that upon inclination, the reduction of the solid-liquid free energy dominates over the increment of the liquid-gas free energy associated with shape deformation. In the second case, the downhill CA grows and the uphill CA remains the same upon inclination. Advancing pinning and receding depinning are noted for the tilted drop at rest. The free energy analysis indicates that upon inclination, the decrease of the liquid-gas free energy compensates the increment of the solid-liquid free energy. The experimental results are in good agreement with those of simulations.

Droplet Compression and Relaxation by a Superhydrophobic Surface: Contact Angle Hysteresis

In this article, the contact angle hysteresis (CAH) of acrylic glass is experimentally and theoretically studied through the compression-relaxation process of droplets by using a superhydrophobic surface with negligible CAH effect. In contrast to the existing technique in which the volume of the droplet changes during the measurement of CAH, this procedure is carried out at a constant volume of the droplet. By observing the base diameter (BD) and the contact angle (CA) of the droplet during the compression-relaxation process, the wetting behavior of the droplet can be divided into two regimes, the contact line withdrawal and the contact line pinning regimes, depending on the gap thickness (H) at the end of the compression process. During the compression process, both regimes possess similar droplet behavior; the contact line will move outward and the BD will expand while the CA remains at the advancing angle. During the relaxation process, the two regimes are significantly different. In the contact line withdrawal regime, the contact line will withdraw with the CA remaining at the receding angle. In the contact line pinning regime, however, the contact line will be pinned at the final position and the CA will decline to a certain value higher than the receding angle. Furthermore, the advancing pinning behavior can also be realized through a successive compression-relaxation process. On the basis of the liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of the droplet during the compression-relaxation process; both contact line withdrawal and pinning regimes can also be identified. The results of the experiment and simulation agree with each other very well.

Wetting behavior of a drop atop holes.

Superhydrophobic surfaces generally involve completely nonwetting or partially wetting roughness. Because the contact angle is closely related to the liquid-gas interfacial tension, the shape of the liquid-gas interfaces within the grooves plays a key role in determining the droplet wetting behavior. We consider a droplet with volume, V, atop holes with radius, r, and obtain the analytical expression of the bottom liquid-air shape based on surface free energy minimization. It is found that the bottom shape in terms of the interfacial angle, theta(1), depends on the hole size through V/r(3) in addition to the intrinsic contact angle, theta(*). For a given droplet volume, the smaller the hole size (r(3)/V --> 0), the more flat the interface (theta(1) --> 0). In addition, the flatness of the interface grows with reducing the intrinsic contact angle. Numerical simulations of Surface Evolver are performed to confirm our theory. Moreover, wetting experiments in which the gravity effect cannot be neglected are conducted, and the results are consistent with those by numerical simulations. Our result points out that such wall-free capillarity may be useful in extracting liquid from microfluidic device spontaneously.

Anomalous wetting on a superhydrophobic graphite surface

A superhydrophobic graphite surface has been fabricated through two facile physical steps, peeling and ultrasonicating. Peeling yields micron-scale roughening, and thus a highly hydrophobic surface is obtained. Further ultrasonicating results in a superhydrophobic surface with nanostructure embedded in microstructure. The nanostructure leads to networklike pores on the superhydrophobic film and convective Ostwald ripening is observed. Owing to their distinct resistance to liquid imbibition, contact angle hysteresis on hydrophobic and superhydrophobic surfaces is fundamentally different. Moreover, the adhesive force on a superhydrophobic surface grows with the contact time, and such aging effect is absent on hydrophobic graphite surface.

Capillary Rise in a Microchannel of Arbitrary Shape and Wettability: Hysteresis Loop

Capillary rise in an asymmetric microchannel, in which both contact angle (wettability) and open angle (geometry) can vary with position, is investigated based on free-energy minimization. The integration of the Young-Laplace equation yields the general force balance between surface tension and gravity. The former is surface tension times the integration of cos θ(u) along the contact line, where θ(u) depicts the local difference between contact angle and open angle. The latter comes from the total volume right underneath the meniscus. For the same channel height, multiple solutions can be obtained from the force balance. However, the stable height of capillary rise must satisfy stability analysis. Several interesting cases have been studied, including short capillary, truncated cone, hyperboloid, and two different plates. As the tube length is smaller than Jurins height, the angle of contact will be tuned to fulfill the force balance. While only one stable state is seen for divergent channels, two stable states can be observed for convergent channels. Three regimes can be identified for the plot of the stable height of capillary rise against the channel height. The higher height dominates in the short channel regime, while the lower height prevails in the tall channel regime. However, both solutions are stable in the intermediate regime. Surface Evolver simulations and experiments are performed to validate our theoretical predictions. Our results offer some implications for water transport to the tops of tall trees. A small bore at the uppermost leaf connected to a larger xylem conduit corresponds to a convergent channel, and two stable heights are possible. The slow growth of the tree can be regarded as a gradual rise of the convergent channel. Consequently, the stable height of capillary rise to the top of a tall tree can always be achieved.

Advancing and receding wetting behavior of a droplet on a narrow rectangular plane

The contact angle (CA) measurements are generally performed on a large planar surface of a specific substrate with the width larger than the droplet size. In this study, the contact angle hysteresis on a narrow rectangular plane with a width smaller than the droplet size is experimentally studied through the inflation–deflation process by the needle–syringe method. The inflation process by stepwise addition of the liquid to the droplet leads to the contact line advancing outwardly along the major axis with advancing angle (θa). Although the droplet width is constrained by the edge of the plane, the CA along the minor axis (θw) increases and its value is greater than θa (θw > θa). Deflation process by stepwise withdrawal of liquid from the droplet results in the contact line retracting inwardly along the major axis as the CA reduces to receding angle (θr). In the meantime, the CA along the minor axis decreases as well. Both advancing and receding angles acquired from the narrow rectangular plane are confirmed with those obtained form the typical large surface of acrylic glass. On the basis of free energy minimization and liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of droplet on the narrow rectangular plane during the inflation–deflation process. The results of experiment and simulation agree with each other very well.