The pull-off test for viscoelastic soft solids

industrial collaborators:	Unilever
academic collaborators:	ESGI59
initiated :	2008/01/28
last updated:	2010/05/25

Study group report 2007: the pull-off test (Unilever)
This is the final report on the problem about the pull-off test for viscoelastic soft solids, brought to ESGI59 by Unilever. Click on the link at the bottom to download the full report as a pdf document.

Report author
Sunny Chiu-Webster (University of Nottingham)
Rosemary Dyson (OCIAM)
David Parker (University of Edinburgh)
Dominic Vella (DAMTP)
Robert Whittaker (University of Nottingham)
David Allwright (Industrial Mathematics KTN)

Executive summary
The adhesive interactions between adhesive soft solids covered with viscoelastic film are crucial for a number of problems related to biological systems like cell adhesion and mucoadhesion. The mucoadhesive interactions are of particular interest to Unilever since most Unilever products interact with mucosa substrates in some way. Foods and beverages interact with oral mucosa including salivary films; this interaction influences taste, mouthfeel and flavour release. Further down the gastrointestinal tract mucoadhesive interactions condition nutrition uptake and to a certain extent enable food digestion. In oral care products mucoadhesion is an important aspect of products' functionality.

Generally mucoadhesion can be described as an interaction of solid or semisolid particles or even liquid droplets with a mucosa substrate that can be defined as a "thick" (about 0.1–200 μm) proteinaceous film. Such adhesive interaction is different from the interaction with solid surfaces, even soft ones. The complexity stems form the fact that several contributions can be identified within the adhesive contact or during rupture of such an adhesive contact, including

1. the adhesive force;
2. the extension of the viscoelastic film that leads to the formation of one or several filaments and their subsequent necking failure;
3. interfacial tension effects (usually small due to low interfacial energy between two water-based phases, e.g. bound and unbound layers) that lead to a capillary effect;
4. the viscoelastic deformation of the substrates and soft bodies.

Motivation for theoretical analysis
In many instances it is required to perform an in-vitro assessment of the mucoadhesive properties of materials that are subject to a screening assay. One of the screening methods is a pull-off test using the AFM. The result of the test is a graph of force against indentation and separation. In the experiment one can vary the speeds of approach and retraction, applied load, dwell time, geometry of interacting surfaces, solvent etc.

The graph of force against separation combines all the effects listed above and does not allow the direct extraction of the parameters of interest, e.g. the extensional viscosity of the proteinaceous mucosa layer or the thickness of this layer. Even the adhesive energy between the mucosa itself and the probe is not measured directly. Since pulloff measurements are dynamic, the whole process can be described using differential equations, some of which are non-linear.

Therefore from Unilever's point of view the main purpose of the theoretical analysis is to establish a method for extracting the parameters of interest from fitting and modelling of an experimental force-separation curve, provided that the theoretical model is adequate for the given experimental conditions. In addition, the theoretical model could provide some physical insights into which contributions are dominating the interaction.

Unilever hopes that the theoretical analysis of a force curve should provide following information:

• adhesive energy (for each of the interfaces - substrate1/film1, film1/film2, film2/substrate2);
• elastic parameters of the viscoelastic substrate;
• viscoelastic parameters of a thin polymer or proteinaceous film adjacent to a viscoelastic substrate;
• thickness of this thin polymer or proteinaceous film.

Unilever's overall aim is to develop a single assay test for mucoadhesive interaction, without needing separate experiments to disentangle the various contributions.

Definition of the problem and experiment
Consider two cases; (i) a viscoelastic solid sphere interacting with a flat mucosa substrate; and (ii) an elastic solid sphere coated with a viscoelastic polymer layer of thickness d interacting with a mucosa substrate. Both cases are illustrated in Figure 1.

The experiments conducted by Unilever suggest that the rupture of the adhesive coating happens in two stages. The first stage is hypothesised to be a rupture of a true adhesive contact, followed by a second stage that is due to the stretching of the polymer or proteinaceous filament. One has to consider also a number of physicochemical phenomena such as hydrodynamic drag, time-dependent adhesion, nonlinear load-dependent adhesion as well as capillary adhesion aggravated by elasto-capillary balance and necking failure. A schematic illustration of rupture, together with a corresponding force curve are presented in Figure 2. The time-dependent contact adhesion effects are related to the dynamics of the polymer chains in the gap between the surfaces. The nonlinear load effects stem from the fact that the contact adhesive interaction can vary across the contact area following the nonuniform stress distribution; in this way polymer properties that are highly susceptible to the pressure or stress lead to a radial distribution in adhesive energy as well as a dependency on applied load. The elasto-capillary force arises from the extension of the viscoelastic polymer bridge that has been formed during the contact.

Click on the link below to view the full report.

 

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	The pull-off test for viscoelastic soft solids
»	Study group report 2007: the pull-off test (Unilever)

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