The idea that society is governed by laws as precise as those of physics has long been a popular hypothesis for the explanation of social behaviour. Contributions to such ideas have been made by Descartes, the followers of the Cartesian system, Immanuel Kant and Auguste Comte.1 Science explores the ordered patterns which are a feature of nature, but these ordered patterns need not necessarily be confined to biological and physical systems.2

In human behaviour there are many instances of phenomena that simulate the physical. Examples include consistent asymmetry, similar to chirality in chemistry, the turning of the head to the right in the last weeks of gestation, and during kissing as an adult.3 There is also the phenomenon of synchrony whereby audiences clap in unison, people walk in time across bridges,4 or women living in the same house menstruate at the same time.

Perhaps the complex way in which people or groups interrelate may be more readily understood if their behaviour is seen to be akin to that of physical interactions. Varying topics may engender agreement, or disagreement. Where there is agreement there is harmony, even sympathy. Where there is disagreement, with antipathetic attitudes and views, a division is created which forms a barrier, an interface.

The term interface has wider usage in science than in common parlance. An interface is a common boundary between two bodies, molecules, liquids or phases, kept apart by interfacial tension forces. Diverse interactions occur in chemical systems, organisms and society at the areas of contact and interface between separate and distinct phases or formations.5 Phases that are soluble, one in the other, will mix, and there is little or no surface tension between the phases. By contrast, other physical entities are immiscible.

It can be helpful in dealing with interactions between people or groups to see these reactions as being similar to those between two physical relatively immiscible systems, for example oil and water. The hypothesis is that behavioural or functional processes have similarities to such a physical phenomenon.

A system is a dynamic assemblage of items which are connected, associated or interdependent, and which form a complex unity, e.g. chemical, cellular or a group of individuals. In a liquid system, each molecule in the interior is attracted on all sides to other molecules in its immediate vicinity, and the forces are balanced. Where there are two immiscible fluids in contact, the outermost layers of substances are in a different state from those in the interior. Molecules near the boundary or surface behave differently because the forces of attraction act only from one side. The resultant of these forces points towards the inside of the liquid, perpendicularly to the surface. The surface is under pressure and there are interfacial energies or turbulence created between phases, whether these be liquid/liquid, liquid/gas, solid/liquid or other combinations of phases. Moving away from the interface into the homogeneous interior, the tension pressure declines as a function of distance from the interface, with tension maximal at the interface and minimal in the centre of the phase. A change in the nature of one or both of the phases may reduce the pressures creating the interface, and hence allow a degree of mixing. This change may be chemical or by the use of a detergent, surfactants or amphipath. Chemical change brings the two phases into solution when the reaction creates a chemical that is miscible in the other phase. An example of this is the conjugation of lipid-soluble compounds, e.g. bilirubin with glucuronic acid, to make a more water-soluble product that can be excreted in the bile. The common practice of washing crockery using hot water and detergent to dissolve the grease is an example of the activity of detergents.

A third system between immiscible fluids is the emulsion, where physical force is applied to such a system, and agitation causes one phase temporarily to disperse so that small particles distribute through the other phase, as happens when a mixture of oil and vinegar are shaken to form salad dressing.

An immiscible interface similar to those between different chemical systems may occur at the boundary between individuals or communities where members of populations of unsympathetic economic, cultural or religious status are juxtaposed. These interfaces may also be related to age, size, gender, employment and educational status, although sometimes these distinctions are not obvious to the outsider. The tension and creation of an interface may apply to one facet of the relationship, whereas other elements may be totally compatible and free of tensions.

What makes for tensions at a social interface? Essentially, reluctance or inability to understand the concerns, beliefs and activities of the adjacent other individual or community. Within a community, the further from the interface the lower the tension, as with a liquid/liquid system. Each individual in the community is attracted on all sides to others in the immediate vicinity, the forces are balanced and the individuals feel comfortable with the community characteristics and beliefs, whereas at the interface these characteristics can be juxtaposed to incompatible attitudes and characteristics. The outermost layers of communities or societies are thus under different stresses from those in the interior. As in the chemical model, there is a relationship between concentration of people or views at the interface. Through the resultant pressures near the boundary or surface, individuals or groups, leaders and vociferous proponents of the beliefs and attitudes of the community behave differently because the forces of attraction to their community now act from only one side. For many, it is more comfortable to be in the homogeneous phase than at the interface. Examples are individuals grouped together, as in a crowd or demonstration facing an equally resolute group holding different views. The interface between the two groups is full of tension and turbulence.

Breakdown of the interface has biological and social significance. Changes in attitude may alter the interface which created the social divisions. A further reducer of the tension can be the introduction of the social equivalent of a detergent, amphipath or surfactant, such as laughter, understanding, good manners or tolerance.6 When a miscible state is achieved, the interface is free from tension, with potentially beneficial consequences.

In medical practice there are instances where doctors find themselves confronted by incompatible groups. Politicians and NHS managers come readily to mind. This phenomenon is a multifaceted system, where there are some aspects where ideas and ideals mix: problems arise where two incompatible groups are each equally convinced that their ideas are correct.

How can the concept of the interface be useful? Change, equivalent to chemical or physical change can be achieved by reduction in anxiety (reducing interface tension), by an increase in communication and hence in trust. Communication can also lead to respect or understanding equivalent to chemical change, a conjugation of ideas or an amphipath. An enforced alliance or resolution of a tension, however, is comparable to an emulsion, which is an unstable system.

For an interface to occur there must be a critical volume of each of the liquids forming the interface. When one phase is reduced in volume, a point is reached when the volume of this phase is such that it will readily to mix into the other phase. The parallel could be fewer managers directing operations, who might then be assimilated into a more harmonious system.

The physical chemist has long understood the creation, characteristics and factors which may influence the interface. The concept is proposed of an interface between individuals and communities and the social surfactants which may reduce the tension at these interfaces. This is however not necessarily rigidly deterministic but capable of self-correction, along lines which also have parallels with physical phenomena.

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