Micro- and nanoscale characterization of hydrophobic and hydrophilic leaf surfaces

Bhushan B., Jung Y.C

Nanotechnology 17 (2006) 2758–2772

Purpose

 

Characterization of micro- and nanoscale topographic features of hydrophobic leaves, and analyze their roll in the superhydrophobic behavior. The study also makes a comparison of hydrophilic leaves with the purpose to understand the importance of low energy surface materials in the improvement of hydrophobic behavior.

Methods

 

Four type of leaves were studied: lotus and colocasia with hydrophobic characteristics and, fagus and magnolia with hydrophilic characteristics. Topographical characterization of the leaves was performed using SEM. Height of the micro and nano features was measured using optical profilometry and AFM in both modes, contact and tapping. Finally wetting properties were determined using a contact angle goniometer.

Key Findings

·         Comparison between hydrophobic and hydrophilic leaves revealed that the later have a thin wax film, which suggests that low energy constituents over this surfaces influence the hydrophobic behavior.

·         Combination of low surface energy materials and roughness are the responsible factors for superhydrophobic behavior.

·         Superhydrophobic behavior combines micro- and nanoscale roughness. However nanoscale roughness is the main responsible for the increase in the contact angle due to the increase of air pockets under the drops.

The “Lotus Effect” Explained: Two Reasons Why Two Length Scales of Topography Are Important

Gao L., McCarthy T.

 

Langmuir 2006, 22, 2966-2967

 

Purpose

Self-cleaning leaves have inspired many researchers to design synthetic routes to obtain superhydrophobic surfaces (surfaces with high contact angles >150° and low contact angle hysteresis <10° in water). This interesting behavior has been attributed to a suitable combination of nano and micro roughness of the surfaces. However, it has not been offered an appropriate explanation of roughness importance to this behavior.

 

Key Findings

·         When a drop rolls over a surface the contact line of the drop usually moves from metastable to metastable state. Over superhydrophobic surfaces the process is different since the contact line does not move, but the drop moves because the liquid-vapor interface descends and wets next post.

·         Receding process of the drop has greater activation energy than the process of advancing.

·         Appropriate combination of nano and micro scale affects the way the drop recedes. This change in the receding process of the drop diminishes the transition state energy between metastable states as the drop rolls

 

Methods

Polymerization of dimethyldichlorosilane in the vapor phase creates microscale roughness over a silicon surface. Nanosacle roughness comes from a second reaction involving polymerization of methyltrichlorosilane. Topographic characterization of the surface was performed by SEM.

Multiscale Roughness and Stability of Superhydrophobic Biomimetic Interfaces

Michael Nosonovsky

 

Langmuir 2007, 23, 3157-3161

 

Purpose

Wettability of a solid by a liquid is characterized by the contact angle, which is the angle between the solid-air and the liquid-air interfaces. The greater the contact angle, the more hydrophobic the material. More recently there has been an increasing interest in superhydrophobic surfaces, which are surfaces with very high contact angles (>150°) and low contact angle hysteresis. This behavior is explained for the presence of air pockets in the valleys between the asperities and the water drop. According to the previous, it is of great importance Formulate a stability criterion for composite interfaces with different roughness profiles, and analyze the implications that different kind of roughness surfaces have over the superhydrophobic character.

 

Key Findings

·         Superhydrophobic surfaces should satisfy the following requirements: they should have a hydrophobic coating, high roughness factors, providing a high contact angle, and the ability to form a composite interface.

·         Superhydrophobic surfaces require having a stable composite interface, a hierarchical roughness structure with nanoscale bumps upon microscale asperities and valleys.

·         Composite interfaces are fragile, since transition to a homogeneous interface is irreversible.

·         Multiscale roughness can help to resist the destabilization, with convex surfaces pinning the interface and thus leading to a stable equilibrium and preventing the filling of gaps between the pillars even in the case of a hydrophilic material.

 

Definitions

·         Angle hysteresis is the difference between advance and receding contact angles.

·         Advance contact angle: is the angle that forms when liquid is added to a surface.

·         Receding contact angle : is the angle that forms when liquid is removed from a surface

 

 

 

Surface-Initiated Polymerization on Self- Assembled Monolayers: Amplification of Patterns on the Micrometer and Nanometer Scale

June 19, 2002

 

Schmelmer U., Jordan R., Geyer W., Eck W., Golzhauser A., Grunze M., Ulman A.

Angew. Chem. Int. Ed. 2003, 42, No. 5

 

 

Purpose of the study.

The authors study advantages in resolution and scale of chemical lithography, a method they developed, in contrast to other patterning techniques such as scanning probe microscopy and microcontact printing. Superior control of the patterning formation is achieved by combining the deposition of functionalized Self Assembled Monolayer (SAM) by chemical lithography and consecutive Surface Initiated Polymerization (SIP), resulting in greater control of pattern formation and amplification of the patterns by creating polymer-brushes in the reaction sites.

 

Methods.

·         SAM formation: Deposition of a SAM of  4’-nitro-1,1’-biphenyl-4-thiol (NBT) on Au(111) was converted into cross-linked 4’-amino-1,1’-biphenyl-4-thiol (cABT) SAM by chemical lithography, resulting in a selective and quantitative reduction of the nitro functionalities to amino groups. The terminal amino groups were then diazotized and treated with methylmalonodinitrile to give a surface-bound monolayer of the 4’-azomethylmalonodinitrile-1,1’biphenyl-4-thiol(cAMBT).

     Phenyldiazenylalkylmalonodinitriles and their derivatives are suitable second-generation initiators for the radical polymerization of a broad variety of vinyl compounds to prepare graft copolymers

·         Polymerization: polymer brushes result from exposing the monolayer of cAMBT to a solution of styrene in toluene at 80C. After 6 hours of reaction, unbound polystyrene was removed from the substrate surface using Soxhlet extraction. 

·         Characterization: FTIR spectroscopy, scanning probe microscopy (SPM), and ellipsometry revealed that the surfaces had homogeneous distributes polystyrene spots 6 nm height, 1.6 μm of diameter and spaced with a periodicity of 2.5 μm

 

Key findings

·         Improvement of lateral resolution of features of the polymer-brush in comparison to other techniques

·         Significant selectivity and uniformity of the process on this large scale in rendering structured polystyrene brushes

·         Chemical lithography is not restricted to any length scale in comparison to scanning probe microscopy and microcontact printing techniques, since allows an efficient patterning of large areas

·         The approach presented in this study allows a greater versatility in the number of choices of surface structures that can be created.

 

Important definitions

·         “Grafting from” polymerization: Type of film fabrication in which the film grows by exposing a rich-initiator surface to a solution of monomer.

·         Chemical lithography: Nanopatterning technique that utilizes specific reactions between the SAMs and electron beams.