Mechanism of Lipid Nanodrop Spreading in a Case of Asymmetric Wetting

Abstract

Using the surface enhanced ellipsometric contrast microscopy, we follow the last stage of the spreading of egg phosphatidylcholine nanodroplets on a hydrophilic substrate in a humid atmosphere, focusing on the vanishing trilayer in terraced droplets reduced to coexisting monolayer and trilayer. We find that the line interface between them exhibits two coexisting states, one mobile and one fixed. From there, it is possible to elucidate the internal structure and the spreading mechanism of the stratified liquid in a case of asymmetric wetting, i.e., where the lipid film is made of an odd number of leaflets.

A diffusive ink transport model for lipid dip-pen nanolithography

“…This leads to the well-known dependence of the spreading speed on time as t −1/2 , 51 even in monolayer spreading. 11 Within this growth-spreading mechanism one may expect irregular-shaped circumferences on circular patterns as reported by Manandhar et al 10 However, since lipid spreading and rearranging are not anisotropic, 25 i.e. 73 In L-DPN, the concentration is dynamically increased below the tip at the meniscus/substrate interface, lipids rearrange and spread over the surface keeping fluid state.…”

“…72 More recently studies of Verma et al include an elastic membrane term that competes with the surface roughening upon spreading, thereby changing the spreading kinetics. This is shown in the membrane spreading studies of Mohamad et al 25 in which the spreading monolayer is fed without a radial symmetry but the monolayer that covers the surface progresses outwards in a 2-D spreading while keeping a circular shape. Here, the spreading environment is the air/lipid/hydrophilic substrate.…”

“…In contrast, concerning height growth, increasing humidity promotes lipids to be displaced from additional membrane stacks into the base membrane through dislocation places, 25 so the equivalent 'diffusion D' along the height growth actually decreases with increasing humidity, relative to the flow rate (the ratio aα/2D increases). When ink mobility and surface mobility are increased simultaneously, ν does not change.…”

“…The fact that n slightly decreases with increasing humidity for RH > 36% indicates that the ink flow rate is getting more enhanced, yet weakly, over surface diffusion. In contrast, concerning height growth, increasing humidity promotes lipids to be displaced from additional membrane stacks into the base membrane through dislocation places, 25 so the equivalent 'diffusion D' along the height growth actually decreases with increasing humidity, relative to the flow rate (the ratio aα/2D increases). Thereby, the net height growth rate n decreases with increasing humidity.…”

“…Upon transition to the substrate surface, lipid rearrangement 14 and membrane self-healing 50,69 are the mechanisms for the lipid membrane assembly; additionally, lipids can transfer from multilayer membranes through dislocation places, feeding the layers below, while the base layer that covers the surface progresses outwards keeping a circular shape. 25 In L-DPN, membrane formation is expected to follow the common phospholipid bilayer spontaneous self-assembly in which single amphiphiles self-organize excluding some water from their hydrophobic molecule parts leading to a decrease in water molecules order (entropy increase driving force). 70 The mechanism of membrane formation from a water containing vessel (meniscus) suggests that in membrane formation during L-DPN some of the meniscus water can be kept, mainly intralayers, that will provide the membrane stacks fluidity.…”