The Transitional Wettability on Bamboo-Leaf-like Hierarchical-Structured Si Surface Fabricated by Microgrinding

At a Glance

Metadata	Details
Publication Date	2022-08-22
Journal	Nanomaterials
Authors	Ping Li, Jinxin Wang, Jiale Huang, Jianhua Xiang
Institutions	Guangzhou University
Citations	4
Analysis	Full AI Review Included

Executive Summary

This research details an eco-efficient, one-step microgrinding method to fabricate Bamboo-Leaf-like Hierarchical-Structured (BLHS) surfaces on monocrystalline silicon (Si), achieving a stable transitional hydrophobic state without chemical modification.

Core Achievement: Successfully transitioned the intrinsic hydrophilic Si surface (Contact Angle, CA < 60°) to a stable transitional hydrophobic state (CA up to 97°) purely through surface texturing.
Fabrication Method: Utilized one-step ultraprecision microgrinding with diamond wheels (SD60-SD3000) to create hierarchical structures (micro-grooves 0.1-10 µm, nanostructures 50-100 nm).
Wetting Stability: The hydrophobic state demonstrated high stability; the CA was reduced by only 2° after high-velocity droplet impact (v=1.0 m/s, Weber number We=21).
Adhesion and Manipulation: The unmodified BLHS exhibits high adhesion, enabling droplet splitting. When coated with low surface energy material (PFPE), the surface becomes superhydrophobic (CA > 135°) and enables rapid droplet self-suction into a micro-tube within 0.1 s.
Mechanism Validation: A new fractal wetting model, based on the measured fractal dimension (D_β 2.06-2.54), accurately explains the stable transitional wetting state observed on the BLHS surfaces.
Industrial Relevance: The method is compatible with hard and brittle materials, offering a durable, coating-free alternative for creating functional Si surfaces for microfluidics and heat transfer applications.

Technical Specifications

Parameter	Value	Unit	Context
Workpiece Material	Monocrystalline Silicon (Si)	N/A	Hard and brittle material
Fabrication Method	Diamond Microgrinding	N/A	One-step forming process
Grinding Wheel Speed (N)	3000	rpm	CNC Grinder (SMART B818)
Feed Speed (v_f)	2000	mm/min	Grinding condition
Depth of Cut (a)	1.5	µm	Fine grinding (X-direction)
Diamond Grain Size Range	SD60 to SD3000	N/A	Used to vary structure scale
Static Contact Angle (CA) Range	77° to 97°	Degrees	Unmodified BLHS surfaces
Superhydrophobic CA	> 135°	Degrees	PFPE-coated BLHS surface
Droplet Volume	3	µL	DI water used for wettability tests
Droplet Impact Velocity (v)	1.0	m/s	Stability test (We=21)
CA Change Post-Impact	2°	Degrees	Indicates stable wetting state
Solid Surface Roughness (S_a)	0.6 to 7.2	µm	Varies inversely with grain size (SD3000 to SD60)
Fractal Dimension (D_β) Range	2.06 to 2.54	N/A	Characterizes hierarchical complexity
Microstructure Depth Range	0.1 to 5	µm	Main groove depth
Nanostructure Depth Range	50 to 100	nm	Secondary structure depth
Droplet Self-Suction Time	0.1	s	Into a hollow micro-tube (PFPE modified BLHS)

Key Methodologies

The BLHS surfaces were fabricated and characterized using ultraprecision machining and advanced metrology techniques to quantify the hierarchical structure and wetting performance.

BLHS Fabrication:
- Monocrystalline Si chips were used as the base material.
- A CNC grinder (SMART B818) was employed for one-step microgrinding using resin-bonded diamond wheels (SD60 to SD3000).
- The grinding path was linear, with fixed parameters: N = 3000 rpm, v_f = 2000 mm/min, and a = 1.5 µm, using water as the coolant.
Surface Morphology Characterization:
- 3D Topography: Measured using a White Light Interferometer (WLI: BMT SMS Expert 3D).
- Micro/Nanostructure Imaging: Performed using Scanning Electron Microscopy (SEM: TESCAN MIRA4).
- Roughness Analysis: Arithmetic mean values (R_ax, R_az, S_a) and Abbott-Firestone curves were calculated to characterize roughness anisotropy and depth distribution.
Fractal Feature Extraction:
- The 3D box-counting method was applied directly to the WLI data to calculate the 2D fractal dimension (D_β) of the solid surface, providing a quantitative measure of multi-scalarity.
Wetting Property Measurement:
- Static CA: Measured using a Dataphysics OCA40 Micro with 3 µL DI water droplets at 25 °C.
- Chemical Analysis: Energy-dispersive X-ray spectroscopy (EDS) confirmed that the surface chemistry remained Si-based, proving the wettability change was purely structural.
Wetting Stability and Adhesion Testing:
- Droplet Impact Test: Droplets were impacted at v=1.0 m/s (We=21) to test the robustness of the hydrophobic state.
- Adhesion Tests: Tensile and compression tests were conducted to quantify the adhesive strength, characterized by tensile and compression ratios (e.g., 40.8% tensile ratio, 66.6% compression ratio).
Surface Modification and Manipulation:
- A photocurable perfluoropolyether (PFPE) coating was applied via UV illumination to achieve superhydrophobicity.
- Droplet manipulation tests (splitting, transfer, and self-suction into a micro-tube) were performed on both unmodified and PFPE-coated BLHS surfaces.

Commercial Applications

The ability to create stable, functional wetting surfaces on Si using a scalable, eco-efficient machining process opens doors for applications where durability and precise fluid control are paramount.

Microfluidic Devices and Lab-on-a-Chip:
- Manufacturing of micro-channels and surfaces for precise droplet steering, splitting, and transfer, crucial for high-throughput biological and chemical analysis systems.
- Utilizing the self-suction capability (0.1 s) for rapid, passive fluid handling in portable diagnostic devices.
Thermal Management Systems:
- Fabrication of durable surfaces for enhanced heat transfer applications (e.g., condensers, heat exchangers). The stable hydrophobic state promotes efficient dropwise condensation, improving overall thermal efficiency compared to filmwise condensation.
Semiconductor and MEMS/NEMS:
- Direct texturing of Si wafers to create functional surfaces for Micro-Electro-Mechanical Systems (MEMS) or advanced electronic packaging, leveraging the high durability of the machined structure over traditional coatings.
Anti-Icing and Anti-Condensation Surfaces:
- Developing surfaces for optical or sensor applications that require resistance to condensation or frost formation, inspired by the natural bamboo leaf’s anti-condensation properties.
Durable Functional Materials:
- Creating environmentally stable surfaces on hard/brittle materials (Si, ceramics) that maintain functionality under extreme conditions (high temperature, acidic/alkaline environments) where polymer coatings typically fail.

View Original Abstract

Stabilizing the hydrophobic wetting state on a surface is essential in heat transfer and microfluidics. However, most hydrophobic surfaces of Si are primarily achieved through microtexturing with subsequent coating or modification of low surface energy materials. The coatings make the hydrophobic surface unstable and impractical in many industrial applications. In this work, the Si chips’ wettability transitions are yielded from the original hydrophilic state to a stable transitional hydrophobic state by texturing bamboo-leaf-like hierarchical structures (BLHSs) through a diamond grinding wheel with one-step forming. Experiments showed that the contact angles (CAs) on the BLHS surfaces increased to 97° and only reduced by 2% after droplet impacts. This is unmatched by the current texturing surface without modification. Moreover, the droplets can be split up and transferred by the BLHS surfaces with their 100% mass. When the BLHS surfaces are modified by the low surface energy materials’ coating, the hydrophobic BLHS surfaces are upgraded to be superhydrophobic (CA > 135°). More interestingly, the droplet can be completely self-sucked into a hollow micro-tube within 0.1 s without applying external forces. A new wetting model for BLHS surfaces based on the fractal theory is determined by comparing simulated values with the measured static contact angle of the droplets. The successful preparation of the bamboo-leaf-like Si confirmed that transitional wettability surfaces could be achieved by the micromachining of grinding on the hard and brittle materials. Additionally, this may expand the application potential of the key semiconductor material of Si.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12162888

References

2020 - Designing “Supermetalphobic” Surfaces that Greatly Repel Liquid Metal by Femtosecond Laser Processing: Does the Surface Chemistry or Microstructure Play a Crucial Role? [Crossref]
2017 - Rapid fabrication of bio-inspired nanostructure with hydrophobicity and antireflectivity on polystyrene surface replicating from cicada wings [Crossref]
2021 - A Unique Bamboo Leaf-Like Nanostructure Based Gas Sensor and Its Potential Application in Indoor Formaldehyde Monitor [Crossref]
2021 - Three-dimensional capillary ratchet-induced liquid directional steering [Crossref]
2014 - Fabrication of superhydrophobic surface with hierarchical multi-scale structure on copper foil [Crossref]
2017 - A lotus-leaf-like SiO2 superhydrophobic bamboo surface based on soft lithography. Colloids Surfaces A: Physicochem [Crossref]
2017 - Durable, high conductivity, superhydrophobicity bamboo timber surface for nanoimprint stamps [Crossref]
2008 - Petal Effect: A Superhydrophobic State with High Adhesive Force [Crossref]
2001 - Mechanisms for the surface colour protection of bamboo treated with chromated phosphate [Crossref]