The ability to control
crystal nucleation through the simple addition
of a nucleating agent (nucleant) is desirable for a huge range of
applications. However, effective nucleating agents are known for only
a small number of systems, and many questions remain about the mechanisms
by which they operate. Here, we explore the features that make an
effective nucleant and demonstrate that the biological material hairwhich
naturally possesses a chemically and topographically complex surface
structurehas excellent potential as an effective nucleating
agent. Crystallization of poorly soluble compounds in the presence
of hairs from a range of mammals shows that nucleation preferentially
occurs at the cuticle step edges, while a novel microdroplet-based
methodology was used to quantify the nucleating activities of different
hairs. This showed that the activities of the hairs can be tuned over
a wide range using chemical treatments. Analysis of the hair structure
and composition using atomic force microscopy, scanning ion conductance
microscopy, and X-ray photoelectron spectroscopy demonstrates that
surface chemistry, surface topography, and surface charge all act
in combination to create effective nucleation sites. This work therefore
contributes to our understanding of heterogeneous nucleating agents
and shows that surface topography as well as surface chemistry can
be used in the design or selection of universal nucleating agents.