Casein for blocking latex beads was prepared by first mixing 6.183 g of boric acid (Sigma, St Louis, MO, B0394) and 800 mL of Ultrapure water (10977-015, Invitrogen, Carlsbad, CA) with stirring. The pH was adjusted to 8.5 using NaOH. Next, 100 g of casein sodium salt powder (Sigma, St Louis, MO, C8654) was added to the solution with continuous stirring. Using a heat plate, the temperature of the solution was brought to 40°C and held overnight. The following day, the solution was diluted to 1 L using ultrapure water and filtered through a Nalgene 0.2 µm filter device (VWR 89176-982). If the filter clogged, the solution was transferred to a fresh unit until the entire solution was filtered. The concentration of casein in the filtered solution was determined by measuring the residual mass of a 1mL aliquot after drying. The solution was diluted in 100 mM borate to bring the final stock concentration to 6% w/v and aliquoted and frozen at -20°C.