Journal of Sol-Gel Science and Technology 35, 183–191, 2005 c  2005 Springer Science + Business Media, Inc. Manufactured in The Netherlands. Tailoring Spontaneous Pillar Structure Using Phase-Separating Organosiloxane Sol-Gel Systems in Micro-Fabricated Grooves KAZUYOSHI KANAMORI ∗ AND KAZUKI NAKANISHI Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan kanamori@kuchem.kyoto-u.ac.jp kazuki@kuchem.kyoto-u.ac.jp KAZUYUKI HIRAO Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyotodaigakukatsura, Nishikyo-ku, Kyoto 615-8510, Japan hirao@bisco1.kuic.kyoto-u.ac.jp HIROSHI JINNAI Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan hjinnai@ipc.kit.ac.jp Received November 12, 2004; Accepted April 20, 2005 Abstract. A sharp transition to pillar structure has been observed in simple sol-gel systems accompanying phase separation inside rectangular-sectioned open grooves which were fabricated on a silica glass chip. The structural variation was investigated using two contrastive systems; one with a rapid sol-gel reaction and the other with a sluggish reaction. In the slower system, transition from web-like bicontinuous structure to pillar structure, we call it as “web-to-pillar transition”, occurred when the bulk characteristic length  m exceeded the width of the groove D. On the other hand, the transition did not occur in the same condition in the faster system; it occurred when  m became much longer than D. A consistent formation mechanism model was also proposed using a relation between interfacial curvatures and pressure. Keywords: methylsiloxane gel, phase separation, confined geometry, wetting, pillar structure 1. Introduction Silica gels with bicontinuous structure have been ap- plied to analytical devices such as high performance liquid chrornatography (HPLC) [1] and solid phase extraction (SPE) [2] etc. in recent days. This type of monolithic column is synthesized via sol-gel accom- panied by spinodal decomposition [3]. In siloxane sol- ∗ To whom all correspondence should be addressed. gel systems, phase separation can be induced by the growth of siloxane polymer in the presence of water soluble polymers or low molecular polar solvents. An- other important process is mesopore formation inside the silica skeletons [4] developed by phase separation. The mesopore formation is achieved by heat-treatment, with a basic aqueous solution or incorporating surfac- tants in the starting solution. Methylsiloxane gel derived from 3-functional methyltrimethoxysilane (MTMS) etc. is another