Thin surface instability

The stability of flow in open channels has been investigated theoretically from a more macroscopic or hydraulic point of view by several workers (Cl7, D9, DIO, Dll, 14, J4, K16, V2). Most of these stability criteria are expressed in the form of a numerical value for the critical Froude number. Unfortunately, most of these treatments refer to flow in channels of very small slope, and, under these circumstances, surface instability usually commences in the turbulent regime. Hence, the results, which are based mainly on the Ch<5zy or Manning coefficient for turbulent flow, are not directly applicable in the case of thin film flow on steep surfaces, where the instability of laminar flow is usually in question. The values of the critical Froude numbers vary from 0.58 to 2.2, depending on the resistance coefficient used. Dressier and Pohle (Dll) have used a general resistance coefficient, and Benjamin (B5) showed that the results of such analyses are not basically incompatible with those of the more exact investigations based on the differential rather than the integral ( hydraulic ) equations of motion. The hydraulic treatment of the stability of laminar flow by Ishihara et al. (12) has been mentioned already. 

In addition, patterns created by surface instabilities can be used to pattern polymer films with a lateral resolution down to 100 nm [7]. Here, I summarize various possible approaches that show how instabilities that may take place during the manufacture of thin films can be harnessed to replicate surface patterns in a controlled fashion. Two different approaches are reviewed, together with possible applications (a) patterns that are formed by the demixing of a multi-component blend and (b) pattern formation by capillary instabilities. 

Majority of dispersed dyes, used for PETP dyeing, are instable themselves [284, 286]. That is why it may be supposed that increase of dye concentration leads to accumulation of being formed radicals in the thin surface layer of the sample without mixing, as a result of which the rate of chain break rises and suppresses chain photooxidation of polymer. This effect was called by the authors [175] effect of concentration inhibition, which was observed in the case of polycaproamide light stabilization by action dyes. 

In spite of the above-mentioned advantages of UTR processes, practical considerations often impose a choice of resist and substrate that are not fully compatible, resulting in films that are unstable or metastable with finite relaxation time. While thick films (> 300 nm) may be stable or metastable due to gravityfor thin films (<100 nm) intermolecular and surface forces dominate. UTR films are susceptible to both spontaneous thin-film instabilities due to London-van der... 

Ultrasonic nozzles use a metal horn geometry to amplify a small peizoelectric vibration. These vibrations drive surface instabilities along a thin film and generate droplets of very uniform size, but at low flow rates. 

Mukherjee, R., Sharma, A., Steiner, U. Surface instability and pattern formation in thin polymer films. In del Campo, A., Arzt, E. (eds.) Generating Micro- and Nanopattems on Polymeric Materials. Wiley-VCH, Weinheim (2011)... 

Another described method of polymer surface modificahon consists in application of thin film instability enhanced by external electric field. Linear structures were created by the effect of locally limited electric field in thin polymer films heated to fluid temperature. 

Viscosities of non-Newtonian polymers are dependent on extrusion temperature and shear rate, both of which may vary within the coextrusion die. The shear rate dependence is further complicated in that it is determined by the position and thickness of a polymer layer in the melt stream. A polymer used as a thin surface layer in a coextruded product experiences higher shear rate than it would if it were positioned as a central core layer. There are several types of flow instabilities that have been observed in coextrusion. 

Tbe most critical requirement in the surface design is the development of a stable surface. The surface undergoes rearrangement of atoms and molecules in response to the external environment and leads to changes In the chemistry and morphology [61]. The problem of surface Instability Is more severe with the plasma exposed materials where the modification is confined to a very thin layer of few... 

It is important to note that spinodal dewetting " - does not imply that the surface instability and consequent dewetting can occur only in the form of a bicontinuous structure composed of liquid ridges or "hills" and "valleys." The thin-film experiments presented here demonstrate that a rich variety of morphological patterns can evolve. 

U. Thiele, M. Mertig, and W. Pompe, Dewetting of an evaporating thin liquid film heterogeneous nucleation and surface instability, Phys. Rev. Lett, 80, 2869 [1998]. 

S.-Q. Huang and X.-Q. Feng, Spinodal surface instability of soft elastic thin films, Acta Mech. Sin., 24,289 (2008]. 

X.-H. Pan, S.-Q. Huang, S.-W. Yu, and X.-Q. Feng, Interfacial slippage effect on the surface instability of a thin elastic film under van der Waals force, /. Phys. D Appl Phys., 42,055302 (2009],... 

Surface Instability and Pattern Formation in Thin Polymer Films... 

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