# Three-Dimensional Soft Material Micropatterning via Grayscale Photolithography for Improved Hydrophobicity of Polydimethylsiloxane (PDMS)

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## Abstract

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## 1. Introduction

## 2. Background of Hydrophobic Properties

## 3. Materials and Method

#### 3.1. Materials

#### 3.2. Development of PDMS Surface Wettability Modification

#### 3.2.1. Design Parameter of Array of Microstructure

#### 3.2.2. Grayscale Mold Fabrication Method

#### 3.2.3. SU8-10 Mold Fabrication

#### 3.2.4. Double Stamping PDMS Relief Fabrication Method

#### 3.3. Water Contact Angle Measurement

## 4. Results and Discussion

#### 4.1. Hydrophobic PDMS

#### 4.2. Super Hydrophobic PDMS

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**PET mask designs of (

**a**) square cross sectional with square distribution, (

**b**) square cross sectional with hexagonal distribution, (

**c**) circle cross sectional with square distribution, and (

**d**) circle cross sectional with hexagonal distribution.

**Figure 4.**Fabrication steps of SU8 micropillar using (

**a**) SU8-10 microhole mold, and (

**b**) PDMS microhole mold (PDMS–PDMS pattern transfer).

**Figure 6.**Contact angle measurement setup: (

**a**) schematic diagram and (

**b**) image of actual measurement setup.

**Figure 8.**Top view SEM images of: (

**a**) SU8 microhole mold, (

**b**) replicated PDMS micropillar, and (

**c**) water contact angle value for square and hexagonal array distribution.

**Figure 9.**SEM images of: (

**a**) SU8 micropillar, (

**b**) PDMS microhole, (

**c**) PDMS micropillar, and (

**d**) water contact angle values.

**Figure 10.**Micropillars with 80 $\mathsf{\mu}$m width: (

**a**) SU8-10 micropillar master mold, (

**b**) replicated PDMS micropillar, (

**c**) surface profile of the SU8-10 micropillar master mold, and (

**d**) surface profile of the replicated PDMS micropillar.

**Figure 11.**Micropillars with 200 $\mathsf{\mu}$m width: (

**a**) SU8-10 micropillar master mold, (

**b**) replicated PDMS micropillar, (

**c**) surface profile of the SU8-10 micropillar master mold, and (

**d**) surface profile of the replicated PDMS micropillar.

**Figure 12.**Cassie–Baxter super hydrophobic wetting state of the PDMS surface; (

**a**) schematic, (

**b**) actual measurement (minimum 150.8 degrees), and (

**c**) actual measurement (maximum 157.3 degrees).

Pillar Width (μm) | SU8-10 Micropillar Height (μm) | PDMS Micropillar Height (μm) |
---|---|---|

80 | 22.53 | 17.31 |

100 | 47.04 | 45.39 |

200 | 72.24 | 63.33 |

Pillar Width (μm) | Fraction, f = $\frac{\mathit{area}\phantom{\rule{0.222222em}{0ex}}\mathit{of}\phantom{\rule{0.222222em}{0ex}}\mathit{a}\phantom{\rule{0.222222em}{0ex}}\mathit{single}\phantom{\rule{0.222222em}{0ex}}\mathit{pillar}}{\mathit{area}\phantom{\rule{0.222222em}{0ex}}\mathit{of}\phantom{\rule{0.222222em}{0ex}}\mathit{a}\phantom{\rule{0.222222em}{0ex}}\mathit{unit}\phantom{\rule{0.222222em}{0ex}}\mathit{cell}\phantom{\rule{0.222222em}{0ex}}\mathit{of}\phantom{\rule{0.222222em}{0ex}}\mathit{the}\phantom{\rule{0.222222em}{0ex}}\mathit{pattern}}$ | Contact Angle of Rough Surface, ${\mathit{\theta}}_{\mathit{r}}$ $cos{\mathit{\theta}}_{\mathit{r}}=\mathit{f}cos{\mathit{\theta}}_{\mathit{s}}-\left(\right)open="("\; close=")">1-\mathit{f}$ |
---|---|---|

80 | $f=\frac{\pi {j}^{2}}{80+100}=\frac{\pi \left(\right)open="("\; close=")">{18}^{2}}{}180$ | $cos{\theta}_{r}=-0.9742$ |

${\theta}_{r}=166.{96}^{\xb0}$ | ||

100 | $f=\frac{\pi {j}^{2}}{100+100}=\frac{\pi \left(\right)open="("\; close=")">{36}^{2}}{}200$ | $cos{\theta}_{r}=-0.9164$ |

${\theta}_{r}=156.{41}^{\xb0}$ | ||

200 | $f=\frac{\pi {j}^{2}}{200+100}=\frac{\pi \left(\right)open="("\; close=")">{21}^{2}}{}300$ | $cos{\theta}_{r}=-0.9874$ |

${\theta}_{r}=170.{88}^{\xb0}$ |

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**MDPI and ACS Style**

Hamid, I.S.L.A.; Khi Khim, B.; Mohamed Omar, M.F.; Mohamad Zain, K.A.; Abd Rhaffor, N.; Sal Hamid, S.; Abd Manaf, A.
Three-Dimensional Soft Material Micropatterning via Grayscale Photolithography for Improved Hydrophobicity of Polydimethylsiloxane (PDMS). *Micromachines* **2022**, *13*, 78.
https://doi.org/10.3390/mi13010078

**AMA Style**

Hamid ISLA, Khi Khim B, Mohamed Omar MF, Mohamad Zain KA, Abd Rhaffor N, Sal Hamid S, Abd Manaf A.
Three-Dimensional Soft Material Micropatterning via Grayscale Photolithography for Improved Hydrophobicity of Polydimethylsiloxane (PDMS). *Micromachines*. 2022; 13(1):78.
https://doi.org/10.3390/mi13010078

**Chicago/Turabian Style**

Hamid, Intan Sue Liana Abdul, Beh Khi Khim, Mohammad Faiz Mohamed Omar, Khairu Anuar Mohamad Zain, Nuha Abd Rhaffor, Sofiyah Sal Hamid, and Asrulnizam Abd Manaf.
2022. "Three-Dimensional Soft Material Micropatterning via Grayscale Photolithography for Improved Hydrophobicity of Polydimethylsiloxane (PDMS)" *Micromachines* 13, no. 1: 78.
https://doi.org/10.3390/mi13010078