Surface tension is a phenomenon which explains why some objects will float on the surface of a liquid even though they are denser than that liquid. How is it for example that some insects are able to run on the surface of water without sinking? How could it be that a coin might also float on the surface of a liquid? It’s all about surface tension.
The definition of surface tension is the attractive force exerted upon the surface molecules of a liquid by the molecules beneath that tends to draw the surface molecules into the bulk of the liquid. This makes the liquid assume a shape having the lowest surface area. Essentially, it is a property of a liquid. It allows it to resist an external force.
The reason we study surface tension is because the adhesion of liquid foods and food emulsion affects the performance of processing equipment, of packaging and resultant residues also cause fouling and even economic losses.
Forces in Surface Tension
A number of forces are at work within a liquid. The first is a cohesive force whereby like molecules stick to each other due to mutual attraction between them. In a water droplet, surface molecules are pulled inward whereas in the bulk of a water droplet those same water molecules are experiencing a cohesive force in all directions.
The other force is an adhesion force which is a property of different molecules or surfaces to bind to each other.
These two contrasting forces explain why a meniscus of a liquid is concave up or down. When he cohesive force of a liquid is stronger than the adhesive force of a liquid to a wall, the liquid must concave downwards to reduce the area of contact with the surface of the wall. The opposite occurs when the adhesive force of the liquid to the wall is stronger than than the cohesive force of the liquid. the liquid is more attracted to the wall than molecules within its body causing the upward concavity.
One of the measures of surface tension is the contact angle. It is the angle through a liquid where a liquid-vapour interface meets a solid surface.
The Shape Of Droplets
Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the imbalance in cohesive forces of the surface layer. In the absence of other forces, drops of virtually all liquids would be approximately spherical. The spherical shape minimizes the necessary “wall tension” of the surface layer according to Laplace’s law.
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting or wettability is determined by a force balance between adhesive and cohesive forces.
Full wetting occurs when the contact angle for a liquid is zero. partial wetting occurs when the contact angle is between 0 and 180 degrees. A nonwetting situation occurs when the contact angle is 180 degrees and the droplet is literally a ball of liquid with almost infinite contact with a surface.
Values for surface tension
Surface tension is measured usually as dynes/cm or mN/m (preferred).
Water at 10ºC is 74.2 mN/m (Rahman, 1995). This value drops as it becomes warmer. So at 25ºC it is 72.0 mN/m and 67.9 mN/m at 50ºC.
Mercury has a very high surface tension and is 485.5 mN/m at 25ºC. (Butt et al., 2006).
Work Aspects of Surface Tension
Where Δ A is the change in the area of the film. As a general rule, the work done to increase the area of a surface is γΔ A. If work is given in joules and Δ A in square meters, so another valid unit for surface tension is J/m2.
Measurement of surface tension
The measurement of surface tension is performed in different ways. two of the most common, simple and relatively accurate methods of calculating surface tension are measured using the rise of a liquid in a capillary tube (capillary rise method) and the pull on a thin vertical plate partially immersed in a liquid (Wilhelmy plate method)
The Wilhelmy plate method not only measures surface tension but also the contact angle between a liquid and a solid. It uses a clean glass or aluminium plate of known weight, W and width, L. A tensiometer measures the force, F required to pull the plate until it detaches from the surface. When the plate’s surfaces are completely wet, the equation below can be used to calculate the surface tension with accuracy of 0.1%.
γ = F-W/2L
The operating procedure is recorded in ASTM D1331-14 Method C. It requires no buoyancy correction and is suitable for moderate viscosities (1 to 10 Pa S) solution.
The capillary rise method is a very old established method.
In the capillary rise method, surface tension draws liquid up a narrow bore tube in the phenomenon known as capillary action. The thin tube is made of glass. For accurate surface tension measurement, the diameter of the thin tube has to be sufficiently small to ensure hemispherical concave/convex meniscus at the liquid–air interface.
The surface tension γ can be determined by using the equation below: .
γ = rhρg/2cosθ
where, r is the radius of the thin tube; h is the rise/drop in liquid column; ρ is the density of resin; g is the gravity pull and θ is the contact angle.
The decrease of surface tension of a protein solution say with time can be monitored using a number of techniques:
Tensiometer
A typical device for measuring surface tension could be this: a Wilhelmy plate type surface tensiometer (e.g. Kyowa Model CBVP surface tensiometer type A3) equipped with a digital printer and a constant temperature cell compartment. Changes in surface tension of 0.1 dyne/cm should be measured reproducibly.
The vessel containing the solution could be a dish of 5.5 cm in diameter and 1.5 cm in depth. The solution is introduced into the vessel and after standing for about 20 minutes to equilibrate the temperature, a fresh surface is formed by removal of the surface of the solution by decantation. The change of surface tension is recorded every minute for 120 mm after the new surface formation. The sample temperature is usually kept at 25 +/- 0.2ºC.
Similarly, the equilibrium surface tension and interfacial tension measurements are performed using the du Nouy’a ring technique carried out with a K12 Krüss tensiometer (Germany) or with a Cahn Electrobalance. To obtain a value, each solution was prepared by dilution of the main solution and a range of starch concentrations from 10E-3 to 10E-1% was considered. Toluene is used as model oil phase. The ring was burnt directly before each measurement. The reproducibility of the surface/interfacial tension measurements was about 0.05 mN/m.
All the measurements were performed in a temperature controlled cell, at 21.0 +/- 0.1 ºC.
The surface/interfacial tension data, obtained can be fitted using the Szyszkowski equation.
γi = γ0 [ 1 – Bsz ln ({c/Asz} + 1)]
where γ0 is the surface tension for the distilled water and Asz and Bsz are the coefficients of Szyszkowski equation.
Similar measurements are claimed to be made using an adapted Instron. An example exists for ice-cream (Wittinger & Smith, 1987). In this example it requires hanging both a DuNouy surface tension ring and a 10g weight from a 500g tension load cell.
Surface tension can also be measured using the ring method with the Cahn Electrobalance (Cini, 1972; Harkins and Jordan, 1930).
To calculate surface tension, the force required to pull the ring from the surface of the liquid was first converted to apparent surface tension by the following equation:-
Apparent surface tension = force (g) x 980.365 cm/sec2 / 2 x ring circumference
The apparent surface tension value was then multiplied by a correction factor to account for the force needed to support the weight of the liquid clinging to the ring at the break-point. The correction factor (CF) was calculated using the following equation:
CF = 0.725 + [(0.01452*(P/CP2) * (D-d) + 0.4534 – 1.679r/R)1/2
where P = apparent surface tension; R = radius of ring; r = radius of wire of the ring; D = density of lower phase (mix); d = density of upper phase (air); and C = circumference of ring.
True surface tension of mix = apparent surface tension x CF
The Pendant Drop Method
Surface and interfacial tension measurements can be measured optically using the pendant drop shape analysis. The shape of the drop hanging from a needle is determined from the balance of forces which include the surface tension of the liquid being investigated.
The surface or interfacial tension can be related to the drop shape by the equation;
γ = ΔρgR0/β
where γ is the surface tension, Δρ is the density difference between fluids, g is the gravitational constant, R0 is the drop radius of curvature at the apex and β is the shape factor. β can be defined through the Young-Laplace equation expressed as 3 dimensionless first-order equations.
Drop-Weight Method/Stallagmometer
Of all the quick and reasonably reliable methods available, the drop-weight method is perhaps the preferred. A drop-weight method, which is a very simple technique is based on the balance between the weight of the drop and the surface tension force of the sample. It therefore appears attractive.
The fundamental principle of this method is based on the force equilibrium of the surface tension and the weight of the drops. The pendant drops start to detach from the tip when the weight of the drops reach the magnitude of the surface tension. Ideally, the drops should fall off completely from the tip but part of the drops is retained in reality. Hence, a correction factor is normally introduced to the calculation of surface tension.
This technique is not very accurate partly because only a portion of the drop actually falls from the capillary tip; as much as 40% of the liquid may remain attached to the tip of the capillary (Adamson & Gast, 1997).
A force balance equation is used to calculate the surface tension using Middleham’s equation:-
6m/ρπ = 4.097σDc/ρg where m is the mass of the sample.
A newly modified version of the Harkins–Brown correction factor has been employed to improve the accuracy of the drop-weight method (Permprasert & Devahastin, 2005). Various solutes such as sucrose, NaCl and acetic acid solutions have been explored to understand the value of the method and how measures using drop-weight method compare to standard values.
Effect Of Temperature
Surface tension does have some dependence on temperature. As the temperature of a solution rises, the liquid becomes more vapour-like until a critical temperature Tc is reached. At such a point, the surface tension must be equal to zero. On that basis, surface tension of a liquid decreases as the temperature rises.
Two empirical relationships have been derived that attempt to explain the surface tension-temperature relationship. The Eotvos relationship named after the Hungarian physicist (1848-1919) has a form:
γV2/3 = k (Tc-T)
where V = M/ρ
Substituting value of V:
γ = k (ρ/M)2/3 (Tc-T)
The empirical equation proposed by Eotvos and then modified by Ramsay and Shields:
γVm2/3 = k (Tc-T – 6.0)
Vm is the molar volume of the liquid, k is a proportionality factor; for non-polar liquids and equals 2.2 x 10E-7 J/K.
The surface tension of aqueous organic acid solutions also decreases with an increase in both solute concentration and temperature (Alvarez et al., 1997).
Thermodynamic Relationships
ΔH = γ – T (dγ/ dT)C,p .
ΔS = – (dγ/ dT)C,p
The Effect of Solutes
Solutes have a different impact on surface tension. Most nonpolar organic molecules have little or no effect. Sugar is a good example of this type of molecule.
Inorganic salts tend to increase surface tension. polar organic molecules such as alcohols, esters, ethers and similar decrease surface tension as a function of their concentration in solution.
Surfactants have a similar dependence on concentration, but they reach a minimum after which adding additional surfactant will have no effect.
The addition of hydrocolloids decreases the surface tension of solutions to varying degrees. The addition of tapioca starch does not affect the surface tension of a suspension (Permprasert & Devahastin, 2005).
It was also found that the type of acid used to adjust the pH affected the surface tension of aqueous solutions differently although the pH of the solutions was equal. Lord et al., (1997) studied the influence of octanoic acid on the surface tension of its aqueous solution; NaOH and HCl were used to adjust the pH in their work. It was found that the surface tension of the air–water–octanoic acid system was dependent on both octanoic acid concentration and pH; the surface tension of the solution decreased when the acid concentration increased and pH decreased.
Wettability
Wettability of pastes is measured using tensiometers e.g. K100 tensiometer [Krüss (Germany)]. A powdered sample of starch for example in the amount of 0.5g dry mass was introduced into a glass tube equipped with a filter at the bottom. The starch was compacted at the bottom. The tube with the sample was mounted at the balancing system and submerged in a tank with water. After the suspended sample contacted the water, the level of water inside the tube increased as a result of the wetting process and the balance
system recorded the mass increase in time. The mass increase was measured over 600 seconds.
Adhesion And Surface Tension
The adhesive behaviour of edible oils and emulsion has been followed for glass, Teflon (PTFE), low density polyethylene (LDPE), poly ethylene terephthalate (PET) and stainless steel. Food samples were allowed to flow down an inclined substrate and the amount of material left after flow had ceased was measured. It was found that surface roughness, the yield stress of the sample and solid surface tension were the key factors responsible for adhesion. These studies suggested that both the surface forces (surface tension related) and bulk forces (viscosity related) are responsible for food adhesion.
This property indicates how strongly the surface molecules of a liquid/solution are attracted by the adjacent molecules and is a factor affecting stickiness. .
Food Examples
Sucrose solutions have been measured in all sorts of conditions (Reiser et al., 1995).
The rheological behaviour of whey protein solutions has been determined with some accuracy (González‐Tello et al., 1995) using a Krüss tensiometer using the Du Nouy ring method. WPC is a well-known surface-active material and is required to achieve maximum surface activity. Any excess of surfactant (in this case whey protein) in the solution will be shielded by the air–water interface and consequently will not contribute to the surfactant effect (Waltra and
Jenness 1984; Roehl and Jelen 1988; Adhikari et al. 2007).
Values of 46.3 mN/m are recorded for WPC in aqueous solution where the concentration as a weight fraction is less than 0..
Honey is mainly a thick sugary syrup of sugars (70–80%), water (10–20%) and other minor constituents such as organic acids, mineral salts, vitamins, proteins, phenolic compounds and free amino acids. Its rheological properties influence its sensory quality (Oroian, 2013).
Kudra (2003) suggested that pasty materials exhibit stickiness across a region of temperature and moisture (sticky region) while being converted from solution to powders.
In summary, surface tension is the cohesive force that exists between the molecules at the surface of a liquid. It arises due to the imbalance of forces acting on the surface molecules compared to those in the bulk of the liquid. Surface tension influences various natural phenomena and has practical applications in diverse fields, ranging from biology to industry. Understanding and harnessing surface tension has broad implications for the behavior and manipulation of liquids.
References
Adamson, A. W., & Gast, A. P. (1967). Physical Chemistry of Surfaces (Vol. 150, p. 180). New York: Interscience publishers.
Adhikari, B., Howes, T., Shrestha, A., & Bhandari, B. R. (2007). Effect of surface tension and viscosity on the surface stickiness of carbohydrate and protein solutions. Journal of Food Engineering, 79(4), pp. 1136-1143 (Article).
Álvarez, E., Vázquez, G., Sánchez-Vilas, M., Sanjurjo, B., & Navaza, J. M. (1997). Surface tension of organic acids+ water binary mixtures from 20 C to 50 C. Journal of Chemical & Engineering Data, 42(5), pp. 957-960.
González‐Tello, P., Camacho, F., Guadix, E. M., Luzon, G., & González, P. A. (2009). Density, viscosity and surface tension of whey protein concentrate solutions. Journal of Food Process Engineering, 32(2), pp. 235-247
International Critical Tables. (1930). International critical tables of
numerical data, physics, chemistry and technology. Edward Washburn, ed. National Research Council, USA.
Lord, D. L., Hayes, K. F., Demond, A. H., & Salehzadeh, A. (1997). Influence of organic acid solution chemistry on subsurface transport properties. 1. Surface and interfacial tension. Environmental Science & Technology, 31(7), pp. 2045-2051.
Oroian, M. (2013). Measurement, prediction and correlation of density, viscosity, surface tension and ultrasonic velocity of different honey types at different temperatures. Journal of Food Engineering, 119(1), pp. 167-172 (Article).
Permprasert, J., & Devahastin, S. (2005). Evaluation of the effects of some additives and pH on surface tension of aqueous solutions using a drop-weight method. Journal of Food Engineering, 70(2), pp. 219-226 (Article).
Rahman, S. (1995). Food Properties Handbook. CRC Press.
Reiser, P., Birch, G. G., & Mathlouthi, M. (1995). Physical properties. In Sucrose (pp. 186-222). Springer, Boston, MA.
Wittinger, S.A. & Smith, D.E. (1987) Adaptation of Instron to Determine the Surface Tension of ice Cream Mix. J. Food Sci., 52(4) pp. 1117-1119 (Article)
Leave a Reply