Electrowetting-based control of droplet transition and morphology on artificially microstructured surfaces

Electrowetting (EW) has recently been demonstrated as a powerful tool for controlling droplet morphology on smooth and artificially structured surfaces. The present work involves a systematic experimental investigation of the influence of electrowetting in determining and altering the state of a static droplet resting on an artificially microstructured surface. Extensive experimentation is carried out to benchmark a previously developed energy-minimization-based model that analyzed the influence of interfacial energies, surface roughness parameters, and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The EW voltage required to trigger a transition from the Cassie state to the Wenzel state is experimentally determined for surfaces having a wide range of surface parameters (surface roughness and fraction of surface area covered with pillars). The reversibility of the Cassie-Wenzel transition upon the removal of the EW voltage is also quantified and analyzed. The experimental results from the present work form the basis for the design of surfaces that enable dynamic control of droplet morphology. A significant finding from the present work is that nonconservative dissipative forces have a significant influence in opposing fluid flow inside the microstructured surface that inhibits reversibility of the Cassie-Wenzel transition. The artificially structured surfaces considered in this work have microscale roughness feature sizes that permits direct visual observation of EW-induced Cassie-Wenzel droplet transition; this is the first reported visual confirmation of EW-induced droplet state transition.     

Electrowetting-based droplet mixers for microfluidic systems

Mixing of analytes and reagents is a critical step in realizing a lab-on-a-chip. However, mixing of liquids is very difficult in continuous flow microfluidics due to laminar flow conditions. An alternative mixing strategy is presented based on the discretization of liquids into droplets and further manipulation of those droplets by electrowetting. The interfacial tensions of the droplets are controlled with the application of voltage. The droplets act as virtual mixing chambers, and mixing occurs by transporting the droplet across an electrode array. We also present an improved method for visualization of mixing where the top and side views of mixing are simultaneously observed. Microliters of liquid droplets are mixed in less than five seconds, which is an order of magnitude improvement in reported mixing times of droplets. Flow reversibility hinders the process of mixing during linear droplet motion. This mixing process is not physically confined and can be dynamically reconfigured to any location on the chip to improve the throughput of the lab-on-a-chip.     

Transition between superhydrophobic states on rough surfaces

Surface roughness is known to amplify hydrophobicity. It is observed that, in general, two drop shapes are possible on a given rough surface. These two cases correspond to the Wenzel (liquid wets the grooves of the rough surface) and Cassie (the drop sits on top of the peaks of the rough surface) formulas. Depending on the geometric parameters of the substrate, one of these two cases has lower energy. It is not guaranteed, though, that a drop will always exist in the lower energy state; rather, the state in which a drop will settle depends typically on how the drop is formed. In this paper, we investigate the transition of a drop from one state to another. In particular, we are interested in the transition of a "Cassie drop" to a "Wenzel drop", since it has implications on the design of superhydrophobic rough surfaces. We propose a methodology, based on energy balance, to determine whether a transition from the Cassie to Wenzel case is possible.     

Nanostructures increase water droplet adhesion on hierarchically rough superhydrophobic surfaces

Hierarchical roughness is known to effectively reduce the liquid-solid contact area and water droplet adhesion on superhydrophobic surfaces, which can be seen for example in the combination of submicrometer and micrometer scale structures on the lotus leaf. The submicrometer scale fine structures, which are often referred to as nanostructures in the literature, have an important role in the phenomenon of superhydrophobicity and low water droplet adhesion. Although the fine structures are generally termed as nanostructures, their actual dimensions are often at the submicrometer scale of hundreds of nanometers. Here we demonstrate that small nanometric structures can have very different effect on surface wetting compared to the large submicrometer scale structures. Hierarchically rough superhydrophobic TiO(2) nanoparticle surfaces generated by the liquid flame spray (LFS) on board and paper substrates revealed that the nanoscale surface structures have the opposite effect on the droplet adhesion compared to the larger submicrometer and micrometer scale structures. Variation in the hierarchical structure of the nanoparticle surfaces contributed to varying droplet adhesion between the high- and low-adhesive superhydrophobic states. Nanoscale structures did not contribute to superhydrophobicity, and there was no evidence of the formation of the liquid-solid-air composite interface around the nanostructures. Therefore, larger submicrometer and micrometer scale structures were needed to decrease the liquid-solid contact area and to cause the superhydrophobicity. Our study suggests that a drastic wetting transition occurs on superhydrophobic surfaces at the nanometre scale; i.e., the transition between the Cassie-Baxter and Wenzel wetting states will occur as the liquid-solid-air composite interface collapses around nanoscale structures. Consequently, water adheres tightly to the surface by penetrating into the nanostructure. The droplet adhesion mechanism presented in this paper gives valuable insight into a phenomenon of simultaneous superhydrophobicity and high water droplet adhesion and contributes to a more detailed comprehension of superhydrophobicity overall.     

Smart design of stripe-patterned gradient surfaces to control droplet motion

The motion of droplets under the influence of lithographically created anisotropic chemically defined patterns is described and discussed. The patterns employed in our experiments consist of stripes of alternating wettability: hydrophobic stripes are created via fluorinated self-assembled monolayers, and for hydrophilic stripes, the SiO(2) substrate is used. The energy gradient required to induce the motion of the droplets is created by varying the relative widths of the stripes in such a way that the fraction of the hydrophilic area increases. The anisotropic patterns create a preferential direction for liquid spreading parallel to the stripes and confine motion to the perpendicular direction, giving rise to markedly higher velocities as compared to nonstructured surface energy gradients. Consequently, the influence of the distinct pattern features on the overall motion as well as suggestions for design improvements from an application point of view are discussed.     

Dynamics of a droplet imbibing on a rough surface

We consider the imbibition of a liquid droplet of finite size on a rough surface and theoretically show that the imbibition dynamics is significantly slower than the familiar Washburn diffusive dynamics, ～t(0.5). The imbibition does not follow a simple power law. The droplet starts to imbibe with ～t(0.5) dynamics but progressively becomes slower with time. The slower imbibition is mainly attributed to the finite size of the droplet, resulting in a limited capillary driving force as compared to a steady capillary driving force in the case of imbibition from a steady source.     

Mechanics of droplet evaporation on heated surfaces

This paper examines the mechanics of evaporation of liquid droplets in direct contact with heated surfaces whose temperature is below the maximum boiling-rate point. The study leads to a classification of the evaporation mode into three categories (i) where the evaporation is controlled by heat transfer through the droplet, (ii) where conduction in the heating plate is the controlling mechanism, and (iii) the intermediate case, where both effects are of comparable importance. A dimensionless parameter is identified which may be employed to characterize the mode of evaporation. Experimental results indicate the general validity of the theoretical analysis.     

Contact angle measurement on rough surfaces

A new method for the measurement of apparent contact angles at the global energy minimum on real surfaces has been developed. The method consists of vibrating the surface, taking top-view pictures of the drop, monitoring the drop roundness, and calculating the contact angle from the drop diameter and weight. The use of the new method has been demonstrated for various rough surfaces, all having the same surface chemistry. In order to establish the optimal vibration conditions, the proper ranges for the system parameters (i.e., drop volume, vibration time, frequency of vibration, and amplitude of vibration) were determined. The reliability of the method has been demonstrated by the fact that the ideal contact angles of all surfaces, as calculated from the Wenzel equation using the measured apparent contact angles, came out to be practically identical. This ideal contact angle has been compared with three methods of calculation from values of advancing and receding contact angles.     

Particle adhesion force distributions on rough surfaces

The effect of roughness on adhesion force distribution was studied in the gas phase. Spherical gold particles with diameters between 5 and 20 microm were generated in a flame process and glued onto atomic force microscope (AFM) cantilevers directly after. Nanostructured substrates with defined roughness were produced by a dip-coating process. The geometry of the adhering partners was determined by AFM imaging, and the adhesion force was measured with the AFM. Depending on the roughness of the particles and the substrates, three types of distribution functions can be identified; two of them can be explained with a simple model. The obtained adhesion force distributions not only agree with those experimentally recorded in previous studies of commercially important powders (e.g., alumina, toner, and gold on different substrates) but also agree with distributions reported in the literature.     

Contact line mobility in liquid droplet spreading on rough surface

We quantitatively estimate the effect of the substrate roughness on the liquid droplet spreading. Since the droplet size is in the order of millimeters, the surface energy becomes the dominant factor. A nonequilibrium thermodynamics framework [Y.X. Gao, H. Fan, Z. Xiao, Acta Mater. 48 (2000) 863-874] seems feasible for describing the millimeter size droplet spreading on a solid substrate. Within the framework, there are two system constants, namely the mobilities of liquid/air surface and the triple joint contact line that need to be determined from experimental testing. In the present paper, we demonstrate the experimental process of determining the mobility of the contact line via a droplet spreading on a steel substrate. Particularly, we obtained the contact line mobility on a steel surface with various roughness values. It is shown that the mobility value is lower for a rougher surface.     

Enhanced hydrophobicity of rough polymer surfaces

Molecular dynamics simulations were used to study the effect of periodic roughness of PE and PVC polymer surfaces on the hydrophobicity. Pillars of different lateral dimensions and heights were derived from flat crystalline surfaces, and the results of nanoscale simulations on the structured surfaces were compared with theoretical predictions of the Wenzel and Cassie equations. Hydrophobicity increased on all rough surfaces, but the increase was greater on the structured PE surfaces because of the larger water contact angle on the flat PE surface than the corresponding PVC surface. Equally sized pillar structures on the two polymers resulted in different equilibrium wetting geometries. Composite contacts were observed on rough PE surfaces, and the contact angle increased with decreasing contact area between the solid and the liquid. Opposite results were obtained for rough PVC surfaces; the contact angle increased with the solid-liquid contact area, in agreement with Wenzel's equation. However, the composite contact was observed if the energies of the wetted and composite contacts were almost equal. Good agreement was obtained between the simulated contact angles and equilibrium droplet shapes and the theories but there were also some limitations of the nanoscale simulations.     

Diffusive dynamics on multidimensional rough free energy surfaces

The dynamics of processes relevant to chemistry and biophysics on rough free energy landscapes is investigated using a recently developed algorithm to solve the Smoluchowski equation. Two different processes are considered: ligand rebinding in MbCO and protein folding. For the rebinding dynamics of carbon monoxide (CO) to native myoglobin (Mb) from locations around the active site, the two-dimensional free energy surface (FES) is constructed using extensive molecular dynamics simulations. The surface describes the minima in the A state (bound MbCO), CO in the distal pocket and in the Xe4 pocket, and the transitions between these states and allows to study the diffusion of CO in detail. For the folding dynamics of protein G, a previously determined two-dimensional FES was available. To follow the diffusive dynamics on these rough free energy surfaces, the Smoluchowski equation is solved using the recently developed hierarchical discrete approximation method. From the relaxation of the initial nonequilibrium distribution, experimentally accessible quantities such as the rebinding time for CO or the folding time for protein G can be calculated. It is found that the free energy barrier for CO in the Xe4 pocket and in the distal pocket (B state) closer to the heme iron is approximately 6 kcal/mol which is considerably larger than the inner barrier which separates the bound state and the B state. For the folding of protein G, a barrier of approximately 10 kcal/mol between the unfolded and the folded state is consistent with folding times of the order of milliseconds.     

Solvation forces between molecularly rough surfaces

Surface heterogeneity affects significantly wetting and adhesion properties. However, most of the theories and simulation methods of calculating solid-fluid interactions assume a standard thermodynamic model of the Gibbs' dividing solid-fluid interface, which is molecularly smooth. This assumption gives rise to a layering of the fluid phase near the surface that is displayed in oscillating density profiles in any theories and simulation models, which account for the hard core intermolecular repulsion. This layering brings about oscillations of the solvation (or disjoining) pressure as a function of the gap distance, which are rarely observed in experiments, except for ideal monocrystal surfaces. We present a detailed study of the effects of surface roughness on the solvation pressure of Lennard-Jones (LJ) fluids confined by LJ walls based on the quenched solid density functional theory (QSDFT). In QSDFT, the surface roughness is quantified by the roughness parameter, which represents the thickness of the surface "corona" - the region of varying solid density. We show that the surface roughness of the amplitude comparable with the fluid molecular diameter effectively damps the oscillations of solvation pressure that would be observed for molecularly smooth surfaces. The calculations were done for the LJ model of nitrogen sorption at 74.4 K in slit-shaped carbon nanopores to provide an opportunity of comparing with standard adsorption experiments. In addition to a better understanding of the fundamentals of fluid adsorption on heterogeneous surfaces and inter-particle interactions, an important practical outcome is envisioned in modeling of adsorption-induced deformation of compliant porous substrates.     

Long-lived adsorbate states on metal surfaces

It has been shown recently that the peculiarities of the band structure of a metal can qualitatively influence the electron tunnelling between an adsorbate and a metal surface, the so-called resonant charge transfer (RCT). The presence of a projected band gap along the normal to the surface in the case of Cu(111) has been shown to lead to a blocking of the RCT in the case of Cs/Cu(111), resulting in the existence of a very long-lived excited state. Such long-lived states are potentially very important for surface reaction mechanisms invoking a transient state as an intermediate. Various systems: Cs, model M- negative ion of p pi symmetry, CO adsorbed on Cu(111), are investigated in order to determine the conditions for the blocking of the RCT and the existence of long-lived states.     

Electromagnetic interactions with rough metal surfaces

When surfaces are structured on the scale of the wavelength, we can expect incident light to be strongly modified by the surface. This is especially the case when the surface is metallic. We have developed a formalism for computing these modifications, closely analogous to electron scattering theory, which we briefly review and present some results for optical properties of, and electron energy loss in, colloids. Our main theme is another effect associate with rough or structured metallic surfaces: Surface Enhanced Raman Scattering, or SERS. We model the rough surface by a periodic array of spheres and obtain the correct magnitude for the enhancement and for the frequency shifts observed.