PDMS spreading morphological patterns on substrates of different hydrophilicity in air vacuum and water

Abstract

In paper has been to investigate the morphological patterns and kinetics of PDMS spreading on silicon wafer using combination of techniques like ellipsometry, atomic force microscope (AFM), scanning electron microscope (SEM) and optical microscopy. A macroscopic silicone oil drops as well as PDMS water based emulsions were studied after deposition on a flat surface of silicon wafer in air, water and vacuum. Our own measurements using an imaging ellipsometer, which also clearly shows the presence of a precursor film. The diffusion constant of this film, measured with a 60,000 cS PDMS sample spreading on a hydrophilic silicon wafer is D(f)=1.4x10(-11) m(2)/s. Regardless of their size, density and method of deposition, droplets on both types of wafer (hydrophilic and hydrophobic) flatten out over a period of many hours, up to 3 days. During this process neighbouring droplets may coalesce, but there is strong evidence that some of the PDMS from the droplets migrates into a thin, continuous film that covers the surface in between droplets. The thin film appears to be ubiquitous if there has been any deposition of PDMS. However, this statement needs further verification. One question is whether the film forms immediately after forced drying, or whether in some or all cases it only forms by spreading from isolated droplets as they slowly flatten out.

Negative Printing by Soft Lithography

“…In contrast, for the inked stamp, where there was excess PDMS on the surface, conventional transfer of polymer on the contact region on Au did occur (see the Supporting Information, Figure S2). However, the low molecular weight PDMS also coated on the “open surface” (not in contact with the stamp) by surface diffusion, , driven by the low surface energy of the polymer relative to Au. ,, As a result, the relative thickness contrast of PDMS was small. Using SEED, the contrast between the contact and the contact region for μCP (i.e., inked) and IμCP (inkless) was 2 versus 74, indicating the latter is a better mode of printing on Au (Figure S3 in the Supporting Information).…”

“…30 In contrast, for the inked stamp, where there was excess PDMS on the surface, conventional transfer of polymer on the contact region on Au did occur (see the Supporting Information, Figure S2). However, the low molecular weight PDMS also coated on the "open surface" (not in contact with the stamp) by surface diffusion, 31,32 driven by the low surface energy of the polymer relative to Au. 10,15,33 As a result, the relative thickness contrast of PDMS was small.…”

“…The precursor layer spread relatively quickly, typically at a velocity of ∼30 μm/min at 25 °C for high molecular weight PDMS. 32 It has also been demonstrated that the rate was higher initially 37 and increased with lower molecular weight PDMS, 38 as well as with improved surface interaction. 39,40 As a result, for 20−80 μm gaps at 60 °C, the spreading was too fast to freeze the precursor layer at a reasonably different penetration depth.…”

Impact of Nanoscale Surface Heterogeneity on Precursor Film Growth and Macroscopic Spreading of [Rmim][NTf2] Ionic Liquids on Mica

“…The published observations of interfacial layering of ILs, including recent observations of thin IL films surrounding droplets deposited from methanol solutions, are suggestive of precursor film formation, which has been observed for conventional liquids such as polydimethylsiloxane. − Precursor films are thin liquid films that spread ahead of the macroscopic droplet front on a solid surface and are known to play an important role in the macroscopic spreading of liquids; however, the influence of surface heterogeneity on precursor film formation and growth has been studied very little to date (even for molecular liquids). The negligible volatility of ILs offers a distinct advantage for studying the development of precursor films because the film growth must occur via surface diffusion alone.…”

AFM study of forces between silicon oil and hydrophobic–hydrophilic surfaces in aqueous solutions

“…Silicone oil is well known to be an excellent wetting agent for most materials [3]. However, it was found that a mica surface would meet this requirement, so long as the mica was first wet by water.…”