Hydrodynamic confinement and capillary alignment of gold nanorods.

Controlling the alignment and orientation of nanorods on various surfaces poses major challenges. In this work, we investigate hydrodynamic confinement and capillary alignment of gold nanorod assembly on chemically stripe-patterned substrates. The surface patterns consist of alternating hydrophilic and hydrophobic micrometer wide stripes; a macroscopic wettability gradient enables controlling the dynamics of deposited suspension droplets. We show that drying of residual liquid on the hydrophilic stripes gives rise to spatially localized deposition and alignment of the nanorods. Moreover, a universal relation between the extent of order within the single layers of nanoparticles and the lateral dimension of the deposits is presented and discussed.

Water-induced blister formation in a thin film polymer.

A failure mechanism of thin film polymers immersed in water is presented: the formation of blisters. The growth of blisters is counterintuitive as the substrates were noncorroding and the polymer does not swell in water. We identify osmosis as the driving force behind the blister formation. The dynamics of the blister formation is studied experimentally as well as theoretically, and a quantitative model describing the blister growth is developed, which accurately describes the temporal evolution of the blisters.

Shape of picoliter droplets on chemically striped patterned substrates.

We studied the shape of water droplets deposited using an inkjet nozzle on a chemically striped patterned substrate consisting of alternating hydrophobic and hydrophilic stripes. The droplet dimensions are comparable to the period of the stripes, typically covering up to 13 stripes. As such, our present results bridge the gap linking two regimes previously considered: (i) droplets on single stripes and (ii) droplets covering more than 50 stripes. In line with previous work on markedly smaller water droplets, the exact deposition position is important for the final shape of the droplets. A droplet with its center deposited on a hydrophobic stripe reaches a shape that is different than when it is deposited on a hydrophilic stripe. Experimental results of different droplet configurations on the same surface are in agreement with simulations using the lattice Boltzmann model. In addition, the simulations enable a detailed analysis of droplet free energies and the volume dependence. The latter reveals scaling properties of shape parameters in terms of droplet radius scaled to the period of the stripe pattern, which have remained unexplored until now.

Simulating anisotropic droplet shapes on chemically striped patterned surfaces.

The equilibrium shape of droplets on surfaces, functionalized with stripes of alternating wettability, have been investigated using simulations employing a finite element method. Experiments show that a droplet deposited on a surface with relatively narrow hydrophobic stripes compared to the hydrophilic stripes adopts a strongly elongated shape. The aspect ratio, the length of the droplet divided by the width, decreases toward unity when a droplet is deposited on a surface with relatively narrow hydrophilic stripes. The aspect ratio and the contact angle parallel to the stripes show unique scaling behavior as a function of the ratio between the widths of the hydrophobic and hydrophilic stripes. For a small ratio, the contact angle parallel to the stripes is low and the aspect ratio high, while for a large ratio, the contact angle parallel is high and the aspect ratio low. The simulations exhibit similar scaling behavior, both for the aspect ratio of the droplets and for the contact angles in the direction parallel to the stripes. Two liquids with different surface tensions have been investigated both experimentally and in simulations; similarities and differences between the findings are discussed. Generally, three parameters are needed to describe the droplet geometry: (i) the equilibrium contact angles on the hydrophilic and (ii) hydrophobic areas and (iii) the ratio of the widths of these chemically defined stripes. Furthermore, we derive a simple analytical expression that proves to be a good approximation in the quantitative description of the droplet aspect ratio.

Tuning kinetics to control droplet shapes on chemically striped patterned surfaces.

The typically elongated shape of droplets on chemically microstriped surfaces has been suggested to depend strongly on the kinetics during deposition. Here, we unequivocally establish the importance of impact kinetics by comparing the geometry of pico- to microliter droplets deposited from an inkjet nozzle with those obtained by conventional deposition from a syringe. For large Weber numbers, the strongly enhanced spreading during the impact in combination with direction-dependent pinning of the contact line gives rise to more spherical droplets with a low aspect ratio. The impact energy can be minimized by the prolonged firing of small picoliter droplets to form larger droplets or, as shown in the past, by using high-viscosity liquids. In the first case, the impact energy is absorbed by the liquid already present, therewith reducing the impact diameter and consequently forming markedly more elongated droplets.

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.

Initial spreading kinetics of high-viscosity droplets on anisotropic surfaces.

Liquid droplets on chemically patterned surfaces consisting of alternating hydrophilic and hydrophobic stripes exhibit an elongated shape. To assess the dynamics during droplet formation, we present experimental results on the spreading of glycerol droplets on such surfaces using a high-speed camera. Two spreading regimes are observed. Initially, in what is referred to as the inertial regime, the kinetics is dominated by the liquid and spreading is only weakly dependent on the specific surface properties. As such, liquid spreading is isotropic and the contact line maintains a circular shape. Our results reveal a remarkably long inertial regime, as compared to previous results and available models. Subsequently, in the viscous regime, interactions between the liquid and underlying pattern govern the dynamics. The droplet distorts from a spherical cap shape to adopt an elongated morphology that corresponds to the minimum energy configuration on stripe-patterned surfaces.

Evaporative gold nanorod assembly on chemically stripe-patterned gradient surfaces.

Experimentally we explore the potential of using pre-defined motion of a receding contact line to control the deposition of nanoparticles from suspension. Stripe-patterned wettability gradients are employed, which consist of alternating hydrophilic and hydrophobic stripes with increasing macroscopic surface energy. Nanoparticle suspensions containing nanorods and nanospheres are deposited onto these substrates and left to dry. After moving over the pattern and evaporation of the solvent, characteristic nanoparticle deposits are found. The liquid dynamics has a pronounced effect on the spatial distribution. Nanoparticles do not deposit on the hydrophobic regions; there is high preference to deposit on the wetting stripes. Moreover, the fact that distributed nanoparticle islands are formed suggests that the receding of the contact line occurs in a stick-slip like fashion. Furthermore, the formation of liquid bridges covering multiple stripes during motion of the droplet over the patterns is modeled. We discuss their origin and show that the residue after drying, containing both nanoparticles and the stabilizing surfactant, also resembles such dynamics. Finally, zooming into individual islands reveals that highly selective phase separation occurs based on size and shape of the nanoparticles.

Lattice Boltzmann modeling of directional wetting: comparing simulations to experiments.

Lattice Boltzmann Modeling (LBM) simulations were performed on the dynamic behavior of liquid droplets on chemically striped patterned surfaces, ultimately with the aim to develop a predictive tool enabling reliable design of future experiments. The simulations accurately mimic experimental results, which have shown that water droplets on such surfaces adopt an elongated shape due to anisotropic preferential spreading. Details of the contact line motion such as advancing of the contact line in the direction perpendicular to the stripes exhibit pronounced similarities in experiments and simulations. The opposite of spreading, i.e., evaporation of water droplets, leads to a characteristic receding motion first in the direction parallel to the stripes, while the contact line remains pinned perpendicular to the stripes. Only when the aspect ratio is close to unity, the contact line also starts to recede in the perpendicular direction. Very similar behavior was observed in the LBM simulations. Finally, droplet movement can be induced by a gradient in surface wettability. LBM simulations show good semiquantitative agreement with experimental results of decanol droplets on a well-defined striped gradient, which move from high- to low-contact angle surfaces. Similarities and differences for all systems are described and discussed in terms of the predictive capabilities of LBM simulations to model direction wetting.