Adding surfactants or other adjuvants can increase the effectiveness of pesticides by positively affecting deposition, spreading, sticking or penetrating into the plant. Understanding the properties of adjuvants by measuring the contact angle of an adjuvant creates insight into the performance of adjuvants. Contact angle is a measurement taken in the lab that reveals a solution’s ability to spread out on and wet a surface.
Contact angle measures the angle of a liquid droplet when it is placed on a solid surface, emulating how a product will spread out on a leaf. The lower the contact angle, the more the adjuvant spreads out, the better wetting abilities it has. This measurement helps compare how well products can wet a surface.
1. A surfactant or adjuvant solution is prepared, loaded into a syringe, and placed into a contact angle goniometer.
2. The surface is prepared on the goniometer test platform. This can be glass, metal, quartz, or any other material so long as it is uniform and replicable.
In the Exacto lab, small pieces of Parafilm are used as our standard surface. This allows for a consistent surface that is both replicable as well as disposable to avoid contamination between tests. Parafilm is also an industry-accepted model for a waxy leaf surface, which makes the test more applicable to the real-world usage of Exacto formulations in agriculture, turf and ornamental or industrial vegetation management.
When an agricultural spray is applied to a plant, the droplets deposit onto the leaf surface. These droplets will either bead up or spread out. This is based on factors like temperature, how waxy the leaf is, how rough the leaf is, and the chemicals in the spray mixture. If a drop beads up, it is more likely to roll off the leaf surface together with the active. If a drop spreads out, it is more likely to stay attached to the leaf surface and keep the agricultural chemical on the plant. When a drop of liquid is put on a solid surface, there are three boundaries to consider – liquid – air, air – solid, solid – liquid. Contact angle is measured where these surfaces meet.
Contact angle measures where gas phase, liquid, and solid meet when a droplet is placed on a surface. A lower contact angle spreads the active over a larger surface.
When a droplet stretches out on a leaf, it increases the surface area it covers. The contact angle measures a liquid’s ability to stretch out over a leaf. A lower contact angle predicts more spread-out droplets, leading to better wetting capabilities.
A droplet that beads up on a surface has a very high contact angle. A droplet that spreads out across a surface has a low contact angle.
Water on Parafilm has a contact angle of roughly 93-95 degrees, which is high. Adding a surfactant or wetting agent can reduce the contact angle significantly depending on the type of surfactant and concentration. This also reduces the chance of droplets beading up and bouncing off the plants during an application.
Contact angle measures where gas phase, liquid, and solid meet when a droplet is placed on a surface.
While any liquid with a contact angle smaller than 90 degrees indicates a wetting capability, the lower the contact angle, the better the spreading is. Liquids with a zero-degree contact angle wet a surface completely and contact angles between 10 and 20 degrees are considered excellent wetters.
Contact angle is a very valuable measurement for comparing adjuvant properties. This test, combined with other useful tests like surface tension and Draves Test, provides concrete data on fundamental aspects of a solution. While they do not always correlate to a solution’s ability to wet a surface, they can provide insight to the characteristics of a formulation. Wetting of solid surfaces is a complicated process that depends on a multitude of other factors, and contact angle is a handy way to measure how adjuvants can help an active cover the target area more effectively.
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