Fabrication of Micro-size Re-entrant
Structure by Two Step Imprint Process
Su Hyun Choi*1 and Young Tae Cho#1
Department of Mechanical Engineering1Changwon National University
Changwon-si, Gyeongsangnam-do Republic of Korea 51139
E-mail: ytcho@changwon.ac.kr

Studies on the fabrication of omniphobic surfaces have been actively conducted for thepurpose of providing special functions such as super-hydrophobic, super-oleophobic, antifouling, anti-icing and self-cleaning on the surface. Among them, in order to fabricate anomniphobic surface through imprint lithography process, studied on the fabrication of a reentrant structure where the upper part of the micro-sized pattern is wider than the lower part.
In this paper, the re-entrant structure was fabricated by oxygen inhibition of UV curable resin.
UV curable resins are classified into radical series and cation series as shown in Table 1and Figure 1 shows radical series react with oxygen to form inert material. Since thegenerated inert material, the status of curing could be inhibited. Based on these properties,
PDMS with higher oxygen permeability than resin was used as a mold material.

Figure 1. Polymerization of PUA resin

Table 1. Characteristics of PUA resin

In the experiment, two PDMS (A, B) were used and imprint experiments were carried out
using PDMS mold of PH (pillar honeycomb) pattern in which pillar structures were arranged
in hexagonal arrangement. Figure 2 and Table 2 show the experimental methods andconditions.

Figure 2. Process of imprint experiment

Table 2. Experimental conditions

The experimental results were reviewed by SEM images and contact angle measurements.
Figure 3 shows that the lower part of the pattern was deformed by the pressure during imprintexperiment as a result of using PDMS A. This shows the result of the reversed phase of there-entrant structure. As a result of experiments using PDMS B, Figure 4 identified a pattern
deformation whose upper part of the pattern was wider than the lower, as defined by reentrant structure.

Figure 3. Experimental result (PDMS A)

Figure 4. Experimental result (PDMS B)

The results of comparing the contact angles of the re-entrant structure produced by imprint
experiment with the common pillar structure can be found in Figure 5. In the pillar structure,
the contact angle was 98 ° when the surface treatment was not performed and 112 ° when thesurface treatment was performed to increase the hydrophobic. Whereas, the contact angle
measurement results of the re-entrant structure was 155.7 °, 157.2 °, and 163.8 ° even though
the surface treatment was not performed. With this, to confirmed the excellent superhydrophobic of the re-entrant structure.

Figure 5. Measurement result of contact angle

Acknowledge:
This research was supported by Industrial Technology Innovation Program Through the Ministry of Trade,Industry & Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT) (N0002310) and theMinistry of Trade, Industry & Energy(MOTIE, Korea) under Industrial Technology Innovation Program.
No.20000665, Development of ecofriendly and highly durable surface treatment for superomniphobic substrateon the large area over 4m2.