Get Rich Foams in Cleansing Products with Science-based Approach

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In cosmetics and personal care products, foams are often for show. A rich foam, with plenty of body to it, is perceived as being performant and sophisticated even though it won’t for example, clean your face or hair any better. Sometimes the foam provides a key function, for example: 

• For shaving, the foam is a moisture barrier to stop the wet skin from drying out.
• In some hair-care products, it is a great way to get a low concentration of active spread over a large area.
• In foaming cleansers, the key attribute is that the active cleansing chemicals stay in place for a long period rather than running down the surface.

Whatever the reason, what we tend to want is a ‘rich foam’. So how do we get it? The simple answer is to use the standard blend of SLES/CAPB (Sodium Laureth Sulfate /CocoAmidoPropyl Betaine CocoAmidoPropyl Betaine) because it is low cost and reliable. Each surfactant on its own is OK, though SLES is a bit too bubbly and CAPB is a bit too expensive and the foam perhaps a little too solid. However, the combination is excellent.

But customers are demanding newer, greener surfactants, perhaps “sulfate-free” or whatever is the latest irrational fad. How do we come up with a good blend from scratch? Because science-based formulation is the way to go, what is the science behind a good foaming system?

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Using apps to bring the science to life is the way I go about science-based formulation. It turns out that for foam-related apps the group of Prof. Nikolai Denkov and Prof. Slavka Tchakalova at U Sofia, Bulgaria, has consistently provided the science I have needed. Some recent (2019-20) papers from this group have consolidated a lot of that science and given me the impetus to write this article.

Relevant references to their work can be found in my various apps. I formally record my thanks for their science and for the kind help they have provided over the years.

Let's understand how to get the right quality, long lasting foam in cleansing formulation using science-based apps.

Creating the Right Sort of Foam

There are many ways to create a foam because all it needs is some air, water, energy to mix them up and some surfactant to stabilize the air/water interface . This ease of foaming is, in fact, a problem because it means there are many different ways to test for foamability, many of which are irrelevant to what is needed for a high-quality foam.

Dropping your surfactant mixture into a vessel containing that mixture (Ross-Miles test), stirring things up in a kitchen blender, blowing air through a glass frit, tipping a volumetric cylinder back and forth are all acceptable foaming tests, but they don’t naturally tend to give the sort of rich foam that interest me for this article.

Spraying a foam mix from an aerosol can is probably the best way to see if you can get rich, long-lasting foam, but is not so convenient unless you are in a specialist lab. Spray bottles can also do a good job though I’ve no hands-on experience to confirm this. I rather like the planetary mixer test (otherwise known as a kitchen blender with a wire whisk) because it really can produce the sort of rich foam that interests us. Out of curiosity, I bought a cheap electric latte whisk to see if it could be used, in principle, as a foam test device. Read to the end to see what happened.

Testing Foam Creation Using the Right Test

A hand-driven way to make lots of rich foam very easily is via either a shaving brush or a “foaming net”. Until recently I didn’t understand either because I had missed the key step. With a shaving brush nothing much happens as you wiggle it in the soap. With a foaming net, again it’s not very obvious that much of interest is happening. In each case the trick is to squeeze out the bubbles you’ve created; you get a rich blob of excellent foam.

Because the brush/net is a good approximation to what happens with shampoo in hair (I would argue that the planetary mixer and latte whisk work in the same way), and because many of the other tests just make lots of relatively large bubbles that aren’t what we’re looking for, your hunt for a new surfactant should start by creating your own sub-version of one of these tests.

Obviously if you are going for a spray, you should find a quick way to make aerosol-like or spray-bottle tests, otherwise use your imagination to create a relevant test. Don’t get hung up by the need for an “industry standard test”. I once found a new co-foamer using an impromptu test created with a syringe needle and some micro-titre plates. Similarly, I created a prototype foam test for another application by rifling through our kitchen utensils and finding something that looked as though it should do the trick – which it did; smarter people then took that idea and made it much more scientific. If €10 latte whisk works for you then you have the basis of a simple, high throughput test, which, as discussed below, is exactly what you need.

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Speed and Ease of Foaming

If you want to foam with low concentrations of surfactant then the rule is easy: use an ionic surfactant with a very fast dynamic surface tension (DST) capability.

Let’s start with the need for the ionic surfactant. The Sofia team has shown convincingly that at high surfactant concentrations, you can make plenty of foam with just about any surfactant. But at low concentrations, your water/air interface isn’t fully covered with a surfactant so non-ionics, which use steric stabilization, just aren’t stable. The ionics can have just 30% coverage and give a reasonably stable bubble. The reasons for this, based on consideration of “disjoining pressure”, are discussed below.

Dynamic Surface Tension

For a guide to DST in general, this app is useful:


The DST issue is about how quickly the surfactant can get to the freshly- created surface. If it’s too slow, the bubble will collapse. At low concentrations, there are large differences between different surfactants (though the ionics tend to be faster than the non-ionics). If you wade through acres of hard-core theory linked to fancy surfactant properties, you reach the conclusion that the only thing you can do with DST is to measure it. This is because there is, so far, no theory that lets us optimize for fast, tough bubbles via properties like CMC or surface excess that you can derive from things like Langmuir isotherms.

Analyzing the Behavior of Surfactants

Although I love science-based formulation, there are times when you must acknowledge that “test and see” is the only rational approach. The need for a DST test in turn means that you need a Maximum Bubble Pressure Tensiometer (MBPT) working in a 200-300ms timescale which, amazingly, translates to the real-world 10ms timescale that is relevant to fast bubble making. The reasons for this difference in timescale are described on my Foam-Making web page.

So, you set up your MBPT and try out your different surfactants, blends and co-foamers till you get a reduction to, say, 40mN/m within the 10ms timescale that distinguishes good from bad foamers. If you don’t have an MBPT, well, sorry, life’s hard and you should maybe just work at the higher-concentration limit. For those who do have an MBPT, the temptation is to do lots of measurements of curves over different timescales. This is very interesting, but if we want to screen a lot of potential combinations, doing a single measurement at the 10ms timescale is all you need.

If you are at 50mN/m then the formulation probably won’t be much good, if you are at 40mN/m you are off to a good start. Remember this isn’t my amateur recommendation, this is the conclusion of the high-powered Sofia team.

If, on the other hand, you know that you will have plenty of surfactant, then the Sofia work tells us that it doesn’t much matter what you use – you’ll generate plenty of foam. The APG surfactants, for example, aren’t especially good at low concentrations (they are non-ionics) but are solid performers at higher concentrations

Related Read: Cleansers 101 - Selecting the Right Surfactant

Quality Foam

If your foaming technique is good, then you automatically get a high-quality foam which is defined by two parameters.

• The first is φ the fraction of air in the foam or its opposite, ε, the fraction of water. If you have φ>0.9 (so ε<0.1) you have a relatively dry foam that won’t run all over the place.
• The second is the bubble diameter D.

How these parameters affect the foam are described in my foam rheology app:

Characteristics of Rich Foam

A rich foam should have a high yield stress, i.e. it needs a relatively large force to make it move. It should also have a high viscosity. Fortunately, both depend (approximately) on φ/D, so a dry foam of small bubbles is both viscous and has a high yield stress. What’s not to like? Note that I’m not asking you to measure the foam rheology – that’s a specialist activity. I’m just saying that the science tells you that with small D and large φ you are off to a good start.

Another feature of foam for which there is some interesting science is opacity. A rich, white foam hides whatever is behind it by scattering light. It turns out that the amount of scattering depends strongly on:

• D, with smaller foams being much more opaque.
• ε, with smaller values being (not surprisingly) more transparent

However, the effect of ε over the range of normal foams is relatively small, so for an opaque foam you need a small D. For those who are interested, you calculate a value:


Then for a thickness of foam, L, the:


Both φ and D depend much more on your foam making method than on the surfactant itself, assuming the surfactant can give a reasonable foam. Indeed, the Sofia team has shown that for the planetary mixer the bubble diameter is self-limiting. As it gets smaller, the viscosity goes up and the mixing no longer causes air to get trapped or bubbles to get split.

One area of quality for which I have no advice to offer is whether the foam feels good. Such sensory judgements are super important for end use, but I have (so far!) no scientific language to describe them. We’ll come back to this important point later.

Long Lasting Foam

There are 4 ways for foam to fail.

1. Via random bursting of bubbles when their walls get very thin. You have so many bubbles in a good foam, and the water content stays high enough for long enough that this isn’t a problem, so you need only worry about the other three.
   
   In academic labs you might worry about the “disjoining pressure” which is the internal forces from the surfactants that resist collapse. But there aren’t any obvious systematic differences between the typical surfactants used, so we needn’t worry about this. For those who are very keen, you can explore disjoining pressures here. This explains why ionic surfactants generally give thicker, more stable films than non-ionics and, therefore, why you don’t need so much surface coverage to get an adequately stable foam film. This explains why ionic surfactants generally give thicker, more stable films than non-ionics and, therefore, why you don’t need so much surface coverage to get an adequately stable foam film.


2. If you have lots of oils present, then these can break the foam. We will come to the science of defoaming in a moment.


3. Drainage – When the water runs out of the foam, the bubble walls get super thin and the foam breaks. My experience with the good foams of interest here is that this isn’t really a problem because the rate of drainage depends strongly on bubble diameter, with smaller bubbles draining much more slowly (halving the diameter increases drainage time 8-fold).
   
   If you have large heights of large frothy bubbles then you really have to worry about drainage, and the type of surfactant is important, with a rigid cell wall (discussed next) much more resistant to drainage, for reasons explained on my app page.
   
   
   


4. It is deeply unfair that if you have a range of bubble sizes from small to large, the small bubbles naturally get smaller while the large ones get larger. This is called Ostwald ripening and is caused by the air being higher pressure inside smaller bubbles – so it diffuses to the lower pressure zones in the larger bubbles.
   
   
   
   
   For a given distribution of bubble sizes, the rate of ripening depends, as described in the app, on:
   
   
   • The solubility of the gas in the water through which it has to travel – so CO₂ foams ripen much faster than air foams. This is the reason, famously, why Guinness beer foam is made with nitrogen not CO₂. For the foams discussed here, there’s not much you can do about this.
   
   
   • The quality of the foam wall. If it’s stiff then it’s a strong barrier, if it’s flexible then it’s a weak barrier.
     
     
   It was worked out long ago that merely by adding small amounts of something like myristic acid (a long-chain carboxylic acid) to a foam formulation, the foam wall becomes much stiffer, providing resistance to random bursting, drainage and Ostwald ripening. The acid does nothing in terms of foam creation but migrates quickly to the bubble surface once its formed, kicking out the more flexible surfactant used to create the foam, and makes a more rigid barrier and a longer-lasting foam. It’s no accident that this sort of rigid foam using things like myristic acid is called a Gillette foam in the academic literature – it was the key to creating a stable shaving foam from an otherwise standard dish-washing surfactant blend which, without it, produced a soft Dawn foam.
   
   In general, tougher foam walls are more resistant to being popped by oils. The science of defoamers deliberately designed for popping walls, was for many years dominated by a theory that was good in theory but useless in practice.

It was, again, the Sofia team who showed that the key factor is a hard-to-define (and hard-to-measure) entry barrier. In any case, a lot of the thinking is irrelevant because (again, shown by the Sofia group), the oils tend to be found in the edges (“Plateau borders”) of the bubbles rather than in the walls, so it takes a lot of drainage before the edges are dry enough for the oil to break the bubble.

The pragmatic advice, therefore, is to ignore all the theories of defoaming and make sure you have a slow-draining, tough-walled foam which resists Ostwald ripening. To be slow-draining you need small bubbles, so again the issue is one of how you generate the small-bubbled foam, not the surfactant formulation itself.

Get the Basics Right for Good Foam

Of course, real life is not as easy as this brief survey implies. There are many complicated ingredients in your formulation which can have all sorts of unwelcome effects on the foaming. My concern here is to get you grounded in the basics:

• A 40mN/m DST at 10ms
• Lots of small bubbles
• Relatively dry bubbles
• Fairly tough bubbles that don’t Ostwald ripen too quickly

If that is your starting point, and if you understand why it’s such a great starting point, then you can deal with the complexities one by one. Any formulation that doesn’t start in that region of formulation space is unlikely to be a longer-term success.

With your set of potentially greener or more natural surfactants, hydrotropes, co-foamers, check out those basics first. The MBPT has to be a given (tell your manager they have to get you one). How wonderful your surfactants are, if they don’t give you 40mN/m in a short timescale, they aren’t going to be acceptable unless you know you can use high concentrations. Remember not to spend too much time gathering data on the whole DST curve – the need is for efficient scouting experiments that get you formulations that meet the 10ms criterion.

Test Foaming Capacity of Surfactants

Now, with the simple small bubble generator, just set up a video camera (or, even better, timelapse capture) looking at your mass of bubbles and recording for, say, 5min. It’s obvious if they fall apart quickly or go from an opaque small-bubble foam to a translucent large-bubble one. Don’t try to do smart image analysis – it doesn’t work without the standard, but complicated, trick of a prism in contact with the bubbles. The human eye is going to be good enough to tell the great formulations from the no-hopers.

With a setup like this you only need small quantities to test and can get through a bunch of variants, trying out a few wild ideas if some of your preferred surfactants simply refuse to foam. I once had a mission-impossible surfactant which suddenly went from zero foam to super-foamer via on unexpected co-foamer. As each test only took a few minutes, I could afford to try such wild ideas. If someone complains that your test isn’t “industry standard” ask them to volunteer to do what you are doing via their preferred test. They will quickly see the wisdom of your small-scale tests.

Ultimately you will have to add validation with a standard test as this is often necessary for internal process control or a standard requirement from customers. So, of course, do a few standard tests to make sure you aren’t going to make a brilliant foam that fails a, regrettably, irrelevant standard test.

Related Read: Cleansers 101 - Selecting the Right Surfactant

Good Foam with Feel-good Factor

Because it’s so easy to make a bunch of small samples, you can invite your sensorial expert to try out a big set of alternatives. This isn’t so much to reach conclusions about perfection. Instead the team gets some quick ideas of what combinations of ingredients produce visually good foams with a sensorial yuk factor or with a wow factor. This provides extra guidance through the complexities of foam formulation space and stops you spending time on great foamers that will never be accepted by end users.

If the expert needs some larger quantities to perform more detailed tests, that’s not a problem, but focus on getting data on a large range of small samples first so the team gets an idea of whether your chosen surfactants and additives are nicely in the feel-good space or are hovering round a feel-bad space. If, while doing this, you can work out the science of the feel-good factor then please let us all know, though your IP experts might not approve.

Because the generation of small bubbles is so important, get other members of the team to focus on that issue within the wider context of how the product will be used. This means that your freedom to cope with other formulation challenges increases.

If your product, even with a good surfactant, can only make wet, big bubble foam, it’s never going to look great and it’s much more likely to collapse for a formulation reason. Your Marketing colleagues won’t want to hear that – they just want magic out of a bottle. But if you make them work a bit harder, imagining how the overall product concept will more naturally create small bubbles, then the chances of a successful product have increased strongly. You can start throwing in all those things they like to include for their marketing claims, without totally destroying your nice foam. It’s a win-win.

The Latte Whisk – Refine Your Optimal Foaming System

The little latte whisk was a revelation. I made a 1% solution of a standard, “light” dish washing liquid, put 10ml of it into a champagne glass (I don’t have a lab with measuring cylinders) and whizzed it for a few seconds.

Not only did I get ~90ml of foam but the bubbles were so fine that they had a high yield strength. It was hard to capture with the camera but there is a hole 2cm diameter and about 2.5cm deep in the middle of the foam from where I withdrew the whisk. The hole was still clearly visible, though reduced, after 4min, with only a small amount of drained liquid visible below the foam. There were no obvious traces of ripening till ~5min. Remember, this is with a Dawn-style foam, never meant to produce a solid, long-lasting foam. After 10min most of the water had drained back. The foam volume had not changed much, but the bubbles had ripened considerably. With an additive to reduce ripening, the foam would have lasted much longer.

To formulate your own, optimal foaming system, a day in the lab refining this latte whisk test would be well worth it. The main difficulty would be finding enough measuring cylinders of the right diameter and depth to meet your requirements. If it needs just 10ml of a 1% solution for each test, you could get a lot of comparative work done at very low cost, with objective (bubble volume and time for, say, 5ml of water to appear at a layer in the bottom) and subjective (e.g. depth and longevity of the central hole as an indication of viscosity/yield stress) data plus some sensorial tests.

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