Surface Passivation Method for the Super-repellence of Aqueous Macromolecular Condensates

Solutions of macromolecules can undergo liquid–liquid phase separation to form droplets with ultralow surface tension. Droplets with such low surface tension wet and spread over common surfaces such as test tubes and microscope slides, complicating in vitro experiments. The development of a universal super-repellent surface for macromolecular droplets has remained elusive because their ultralow surface tension requires low surface energies. Furthermore, the nonwetting of droplets containing proteins poses additional challenges because the surface must remain inert to a wide range of chemistries presented by the various amino acid side chains at the droplet surface. Here, we present a method to coat microscope slides with a thin transparent hydrogel that exhibits complete dewetting (contact angles θ ≈ 180°) and minimal pinning of phase-separated droplets in aqueous solution. The hydrogel is based on a swollen matrix of chemically cross-linked polyethylene glycol diacrylate of molecular weight 12 kDa (PEGDA), and can be prepared with basic chemistry laboratory equipment. The PEGDA hydrogel is a powerful tool for in vitro studies of weak interactions, dynamics, and the internal organization of phase-separated droplets in aqueous solutions.


Dynamic contact angle
Contact angle hysteresis is the difference between the contact angle of a droplet sitting on a surface in wetting and dewetting regions.While on perfectly defect-free, ideal surfaces the contact angle θ is a unique value, on a real surface and in dynamic conditions, θ varies between two extremes, θ a and θ r , known as the advancing and receding contact angles, respectively 1,2 .If we imagine a droplet sitting on a flat surface, its shape will be symmetrical.Let's now tilt the surface at an angle α.There is a force, the pinning force, between the droplet and the surface, which will maintain the droplet adhered to the surface.Hence, it will initially not move, but it will start deforming under its own weight (Fig. S7).When a critical value of α is reached, however, the droplet will start sliding.Because of the deformation, the contact angles at the advancing and receding ends will be different: these are θ a and θ r .The stronger the pinning force, the more the droplet will deform before starting to move, hence the larger the difference ∆θ = θ a − θ r .
The values of θ a and θ r can be calculated by a simple force balance 3 .A side-view of the droplet is represented in Fig. S7.The droplet slides as a result of the parallel component of the gravitational force with respect to the surface, F G∥ , which be expressed as where ∆ρ is the density difference between the droplet and the surrounding medium, V the droplet volume, g the gravitational acceleration, and α the tilting angle.
The pinning force F , on the other hand, counteracts droplet motion.This should be derived in principle from the droplet surface tension balance along the whole droplet contact line.In most studies, however, its value is simply determined by the balance of forces at the advancing (f a ) and receding (f r ) ends, multiplied by a geometrical pre-factor, k: area, k = 48 π 3 R offers a good approximation, where R is the droplet radius as seen from the side 5 .By expressing f a and f r in terms of the droplet surface tension γ, we can then rewrite Eq. 2 as At the onset of motion, F G∥ = F , hence: which then can be rearranged as Importantly, we can estimate the retention force using Eq. 1 by measuring the critical tilting angle α, the droplet volume V , the density difference ∆ρ, and knowing the gravitational acceleration g.First, we substitute V ≈ 4πR 3 3 into Eq.5: We note that the relevant dimensionless quantity is the Bond (or Eötvös) number: Thus, that is, the critical tilt angles can only be used as a relative measure of contact angle hysteresis between droplets with similar Bond numbers.We then use the Taylor expansion

S3
of cos θ around θ ≈ 180 • , to rewrite the left-hand side of Eq. 8 as follows: The substitution θ r = θ a − ∆θ yields or Substituting Eq. 11 into Eq.8 yields a quadratic equation for ∆θ: The positive solution for Eq. 12 is ∆θ = (θ a − π) Taking the upper bound of the right-hand side of Eq. 13 yields Photoinitiator O O OH OH

Figure S4 :
FigureS4: PEGDA 12 kDa substrate preparation Pre-treatment (1) Clean glass slides are treated with UV-Ozone for 10 minutes and then (2) coated with a solution containing 3-(trimethoxysilyl) propylmethacrylate (silane coupling agent, red ).The reaction is allowed to complete over the course of 24 hours.Coating (3a) A solution of PEGDA 30 % by weight in water is made (cyan).(3b) At the same time, a solution of photoinitiator 2 % by weight in water is made (orange) and sonicated for 20 minutes at 55 • C to assist solubilization.The PEGDA and photoinitiator solutions are then mixed together at 1:1 volume ratio (green) to yield final concentrations of 15% w/w PEGDA and 1% w/w photoinitiator and then (4) added to a pre-treated glass slide.Curing(5) The drop of the mixture is sandwiched between the pre-treated glass slide and a glass slide coated with RainX ® (yellow ) and then (6) cured under UV light for 1 hour.(7) The top glass slide is then removed to expose the cured PEGDA hydrogel.

αFFigure S7 :
Figure S7: Force balance of a droplet sliding on a tilted surface.With advancing contact angle θ a and receding contact angle θ r

Table S1 :
AC treatment program for electroformation of GUVs