draves test for wetting agents and surfactants

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What Draves Test Reveals About Wetting Agents & Surfactants

Chris Walter, Chemist
Chris Walter, Chemist

How do you know what surfactant and wetting agent formulations can do? Lab tests show what characteristics a formulation has and how they affect tank mix properties.

Draves Test is an in-lab test that can compare how well products can wet hydrophobic surfaces and penetrate hydrophobic solids. The Draves Test is one of many tests run in the Exacto Lab to better understand the properties of our formulations and create better products.

Understanding what goes on in the lab can help users understand how the products they use may act and perform. This helps applicators optimize their tank mixes to achieve ideal results.

Draves Test Method

  1. Dilute adjuvant solution is made and transferred into a graduated (or other transparent) cylinder.
  2. A cotton skein* is then folded twice and cut through the middle to create many equal-length cotton strands.
  3. A standard lead weight attached to a copper hook with a string is then used to weigh the cut skein down and it is dropped into the cylinder.
  4. Timing begins when the weight touches the bottom of the cylinder and ends when the string becomes slack.
  5. Results are measured in seconds.

*Skein: a length of thread or yarn, loosely coiled and knotted. 

What We Learn from Draves Test

Draves Test allows surfactants and other adjuvant products to showcase their “wetting” abilities in comparison to other substances. Wetting refers to the ability of a liquid to coat a solid surface. This capability is essential to optimize plant uptake, ensuring application effectiveness, and minimizing spray runoff. Increasing the wetting capabilities of an application is what surfactants and wetting agents are meant to do.

The cotton skein in this experiment models the plant, showing just how much wetting the adjuvant solution can cause. The more waterlogged the skein becomes in a shorter amount of time, the better the wetting characteristics of the substance. Cotton is mainly composed of hydrophilic cellulose, so it should get wet very quickly. However, a small amount of hydrophobic oils and waxes in the cotton prevent the water from penetrating and adsorbing* to the cellulose. Therefore, the skein will only fall if a surfactant or wetting agent is in the solution.

*Adsorb: (of a solid) hold (molecules of a gas or liquid or solute) as a thin film on the outside surface or on internal surfaces within the material.

cotton skein used for draves test for wetting agents and surfactants

A cotton skein cut through the middle to create many equal-length cotton strands

Why Use Wetting Agents and Surfactants

Hydrophobicity is a common trait found in nature and often presents a real challenge. Waxy plant leaves cause water to bead up and roll off the leaf. The same issue can also be caused by microstructures like tiny hairs or ridges on leaves creating a rough surface and, in turn, hydrophobicity. This poses a challenge for water-based herbicides to get inside the plant to do their job. Products like fungicides only protect areas that are covered by the fungicide. So improper wetting causes unprotected patches on the leaf. Some products need to dry to function, and droplets dry slower than a thin layer of liquid does.

While many tank mixes can benefit from the extra wetting power of a surfactant, it is most important for water-based sprays. Since water has a tendency to bead up and roll off waxy plant leaves, water-based sprays typically experience hydrophobic interaction when applied. Adding an adjuvant with surfactant qualities can mitigate this common issue.

In addition to plant leaves, soils can also be hydrophobic. This occurs when sandy soils dry out and when there are high amounts of non-polar organic matter present. Draves Tests can be applicable to soil hydrophobicity but the most common test to measure soil hydrophobicity is called the Water Drop Penetration Time test. Hydrophobic soils are extremely difficult to wet, and this presents problems in both the turf & ornamental and agriculture industries, where soil water retention is essential to maintain high-quality turf or crops. Soil wetting agents are designed to overcome this hydrophobicity and help irrigation penetrate and get into the soil where it is available for plants.

How Do You Know Which Formulation is Better?

Draves Test doesn’t measure a fundamental force or chemical property. Instead, it is an empirical test that can compare products and solutions consistently and repeatedly. That’s why it is one of several tests used to compare Exacto’s products. However, while a Draves test produces consistent results, like many other tests, some variables cannot be controlled and cause a variance, particularly when the test is performed with smaller solution amounts.

In most situations, Draves produces results that are consistent enough to be statistically significant. Most solutions with test times greater than 60 seconds will show variance in time of less than +/- 10%. Solutions with times less than 60 seconds can sometimes show more variance, but it is usually around 10% of the average time. Variance in the measurements can be caused by skein variability, human variability in cutting and flaying the skein, differences in the solution from test to test, and Critical Micelle Content (CMC) of the test solution.

Critical Micelle Content (CMC) is the concentration of a surfactant in a bulk phase, above which aggregates of surfactant molecules (micelles) start to form.

Draves Tests are an important tool for comparing adjuvant properties. Contact angle and surface tension are other useful tests which provide concrete data on fundamental aspects of a solution, but they do not always correlate to a solution’s ability to wet a surface. Wetting of solid surfaces is a complicated process that depends on a multitude of other factors, and the Draves Test is a useful way to evaluate how adjuvants help accomplish this goal.

Exacto products are developed through rigorous testing that ensures end-users are getting consistent, quality performance. The Draves Test is one useful tool to compare wetting properties of adjuvants. These results, combined with a suite of other tests like contact angle and surface tension, ultimately lead to the development of optimized products that are designed to make a positive impact on spray applications.

The Draves Test follows ASTM D2281-68 guidelines.

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